Is there work underway to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths?

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The Max Planck Institute for Radio Astronomy's press release Something is Lurking in the Heart of Quasar 3C 279; First Event Horizon Telescope Images of a Black-Hole Powered Jet shows a stunning montage of three Event Horizon telescope images at 7, 3 and 1.3 mm wavelengths (43, 86 and 230 GHz) demonstrating how the highest frequency in combination with the planet-sized baselines work together to produce observations at "an extreme 20 microarcsecond resolution", quoting the title of the April 5 2020 Astronomy and Astrophysics paper Kim et al. 2020 Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution.

From How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes? I know that ALMA's receivers can go as far as about 950 GHz. Is there work underway to increase the number of radiotelescope sites around the Earth with circa 1 THz receivers to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths?

Author: Harriet Parsons

Two Hawaii-based telescopes, the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, and the Submillimeter Array (SMA), operated by the Smithsonian Astrophysical Observatory and the Academia Sinica Institute for Astronomy and Astrophysics, have once again combined efforts with the global network of telescopes known as the Event Horizon Telescope. Today the image of Pōwehi, the Black Hole at the Centre of M87, has been shown in new light – specifically polarized light. The polarized light has enabled astronomers for the first time in history to measure polarization, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core.

“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.

On 10 April 2019, scientists released the first ever image of a black hole, Pōwehi, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarized.

A view of the M87 supermassive black hole in polarized light. The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole released in 2019, has today a new view of the massive object Pōwehi at the centre of the Messier 87 (M87) galaxy: how it looks in polarized light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole.This image shows the polarized view of the black hole in M87. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. Credit: EHT

Light becomes polarized when it goes through certain filters. As an example many of us here in Hawaii have polarized sunglasses, in space light can become polarized when it is emitted in hot regions of space that are magnetized. In the same way polarized sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their vision of the region around the black hole by looking at how the light originating from there is polarized. Specifically, polarization allows astronomers to map the magnetic field lines present at the inner edge of the black hole.

The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its centre are one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.

Hilo astronomer Geoff Bower who is the EHT Project Scientist said These beautiful images tell an amazing story of how powerful magnetic fields control the black hole’s appetite and funnel part of its lunch out at nearly the speed of light. Producing these images was an incredible technical achievement from observations around the world to sophisticated image analysis.”

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how jets larger than the galaxy are launched from its central region, which is as small in size as the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarized light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.

This composite image shows three views of the central region of the Messier 87 (M87) galaxy in polarised light. The galaxy has a supermassive black hole at its centre and is famous for its jets, that extend far beyond the galaxy. One of the polarised-light images, obtained with the Chile-based Atacama Large Millimeter/submillimeter Array (ALMA), shows part of the jet in polarised light, with a size of 6000 light years from the centre of the galaxy. The other polarised light images zoom in closer to the supermassive black hole: the middle view covers a region about one light year in size and was obtained with the National Radio Astronomy Observatory’s Very Long Baseline Array (VLBA) in the US. The most zoomed-in view was obtained by linking eight telescopes around the world to create a virtual Earth-sized telescope, the Event Horizon Telescope or EHT. This allows astronomers to see very close to the supermassive black hole, into the region where the jets are launched. The lines mark the orientation of polarisation, which is related to the magnetic field in the regions imaged.The ALMA data provides a description of the magnetic field structure along the jet. Therefore the combined information from the EHT and ALMA allows astronomers to investigate the role of magnetic fields from the vicinity of the event horizon (as probed with the EHT on light-day scales) to far beyond the M87 galaxy along its powerful jets (as probed with ALMA on scales of thousand of light-years). The values in GHz refer to the frequencies of light at which the different observations were made. The horizontal lines show the scale (in light years) of each of the individual images. Credit: © EHT Collaboration ALMA (ESO/NAOJ/NRAO), Goddi et al. VLBA (NRAO), Kravchenko et al. J. C. Algaba, I. Martí-Vidal

The team found that only 0.1% of the theoretical models can explain what the astronomers are seeing at the event horizon. The new observations also revealed information about the structure and strength of the magnetic field just outside the black hole that astronomers didn’t have before.

“Our first glimpse of Pōwehi – a snapshot of the total light intensity – was like seeing the movie poster. Now, with our polarized glasses on, we have front row seats as the film begins. The polarized images show us how black holes do what they do and why we see what we see,” JCMT Deputy Director, Dr Jessica Dempsey states. “Our worldwide and home team pushed every technical, theoretical and observational boundary to achieve this. And we are still in the first minutes of the story. We have so much more to see. Pass the popcorn.”

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, including the JCMT and SMA located on Maunakea, to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.

This allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarized-light image clearly showing that the ring is magnetized.

“The EHT is a one-of-a-kind facility to test the laws of physics in a region of extreme gravity. It gives us a unique chance to look at phenomena we have never studied before,” says EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica, Institute of Astronomy and Astrophysics in Taiwan.

Future EHT observations will reveal even more information about the mysterious region of space near the event horizons of supermassive black holes.The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research, which was coordinated by Mościbrodzka, involved over 300 researchers from multiple organisations and universities worldwide. Simon Radford, Director of Hawaii Operations, Submillimeter Array said “This research showcases the close cooperation between observatories in Hawai’i and elsewhere. The SMA and the JCMT have participated in the EHT for more than a decade. They will continue to play a major role in future EHT observations because of their location, their technology, and the dedication of their talented staff.”

2018 Will Be The Year Humanity Directly ‘Sees’ Our First Black Hole

Black holes are some of the most incredible objects in the Universe. There are places where so much mass has gathered in such a tiny volume that the individual matter particles cannot remain as they normally are, and instead collapse down to a singularity. Surrounding this singularity is a sphere-like region known as the event horizon, from inside which nothing can escape, even if it moves at the Universe’s maximum speed: the speed of light. While we know three separate ways to form black holes, and have discovered evidence for thousands of them, we’ve never imaged one directly. Despite all that we’ve discovered, we’ve never seen a black hole’s event horizon, or even confirmed that they truly had one. Next year, that’s all about to change, as the first results from the Event Horizon Telescope will be revealed, answering one of the longest-standing questions in astrophysics.

The idea of a black hole is nothing new, as scientists have realized for centuries that as you gather more mass into a given volume, you have to move at faster and faster speeds to escape from the gravitational well that it creates. Since there’s a maximum speed that any signal can travel at — the speed of light — you’ll reach a point where anything from inside that region is trapped. The matter inside will try to support itself against gravitational collapse, but any force-carrying particles it attempts to emit get bent towards the central singularity there is no way to exert an outward push. As a result, a singularity is inevitable, surrounded by an event horizon. Anything that falls into the event horizon? Also trapped from inside the event horizon, all paths lead towards the central singularity.

Practically, there are three mechanisms that we know of for creating real, astrophysical black holes.

1. When a massive enough star burns through its fuel and goes supernova, the central core can implode, converting a substantial fragment of the pre-supernova star into a black hole.
2. When two neutron stars merge, if their combined post-merger mass is more than about 2.5-to-2.75 solar masses, it will result in the production of a black hole.
3. And if either a massive star or a cloud of gas can undergo direct collapse, it, too, will produce a black hole, where 100% of the initial mass goes into the final black hole.

Over time, black holes can continue to devour matter, growing in both mass and size commensurately. If you double the mass of your black hole, its radius doubles as well. If you increase it tenfold, the radius goes up by a factor of ten, also. This means that as you go up in mass — as your black hole grows — its event horizon gets larger and larger. Since nothing can escape from it, the event horizon should appear as a black “hole” in space, blocking the light from all objects behind it, compounded by the gravitational bending of light due to the predictions of General Relativity. All told, we expect the event horizon to appear, from our point of view, 250% as large as the mass predictions would imply.

Taking all of this into account, we can look at all the known black holes, including their masses and how far away they are, and compute which one should appear the largest from Earth. The winner? Sagittarius A*, the black hole at the center of our galaxy. Its combined properties of being “only” 27,000 light years distant while still reaching a spectacularly large mass that’s 4,000,000 times that of the Sun makes it #1. Interestingly, the black hole that hits #2 is the central black hole of M87: the largest galaxy in the Virgo cluster. Although it’s over 6 billion solar masses, it lies some 50–60 million light years away. If you want to see an event horizon, our own galactic center is the place to look.

If you had a telescope the size of Earth, and nothing in between us and the black hole to block the light, you’d be able to see it, no problem. Some wavelengths are relatively transparent to the intervening galactic matter, so if you look at long-wavelength light, like radio waves, you could potentially see the event horizon itself. Now, we don’t have a telescope the size of Earth, but we do have an array of radio telescopes all across the globe, and the techniques of combining this data to produce a single image. The Event Horizon Telescope brings the best of our current technology together, and should enable us to see our very first black hole.

Instead of a single telescope, 15-to-20 radio telescopes are arrayed across the globe, observing the same target simultaneously. With up to 12,000 kilometers separating the most distant telescopes, objects as small as 15 microarcseconds (μas) can be resolved: the size of a fly on the Moon. Given the mass and distance of Sagittarius A*, we expect that to appear more than twice as large as that figure: 37 μas. At radio frequencies, we should see lots of charged particles accelerated by the black hole, but there should be a “void” where the event horizon itself lies. If we can combine the data correctly, we should be able to construct a picture of a black hole for the very first time.

The telescopes comprising the Event Horizon Telescope took their very first shot at observing Sagittarius A* simultaneously last year. The data has been brought together, and it’s presently being prepared and analyzed. If everything operates as designed, we’ll have our first image in 2018. Will it appear as General Relativity predicts? There are some incredible things to test:

• whether the black hole has the right size as predicted by general relativity,
• whether the event horizon is circular (as predicted), or oblate or prolate instead,
• whether the radio emissions extend farther than we thought, or
• whether there are any other deviations from the expected behavior.

Whatever we do (or don’t) wind up discovering, we’re poised to make an incredible breakthrough simply by constructing our first-ever image of a black hole. No longer will we need to rely on simulations or artist’s conceptions we’ll have our very first actual, data-based picture to work with. If it’s successful, it paves the way for even longer baseline studies with an array of radio telescopes in space, we could extend our reach from a single black hole to many hundreds of them. If 2016 was the year of the gravitational wave and 2017 was the year of the neutron star merger, then 2018 is set up to be the year of the event horizon. For any fan of astrophysics, black holes, and General Relativity, we’re living in the golden age. What was once deemed “untestable” has suddenly become real.

Is there work underway to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths? - Astronomy

This is nicely done! Using closed-form expressions for the ray intersections is a good improvement, it makes the whole thing run much more smoothly. I posted a similar shader a while ago in this post, where I used 'standard' ray marching to render the shapes. I have since (occasionally) been playing around with rendering implicit surfaces defined by volumetric noise, for some more complex geometry.

It's reassuring that your result looks exactly the same :).

Absolutely! Seems like the math works out :)

I greatly enjoy rendering these kinds of curved spaces, and specifically I try to think of the various types of game concepts that would work well in different types of spaces, so that players can really explore the oddities of such a space. The 3-sphere would work well for some kind of 'pocket universe' game, where there is no artificial boundary but only so much empty space. Gravity can be implemented much like in flat space, so maybe some orbital dynamics can be incorporated as well. Furthermore, it might be cool to have the player start out on a planet, and have the scale of the initial environment be so small that the curvature of the space itself does not even become apparent until later, when the player is more free to travel around away from the planet.

Of course, hyperbolic space already has Hyperrogue in 2D. In full 3D, a game concept where some relatively close-by objects are surprisingly hard to find might be interesting. And the challenge of backtracking is still interesting in 3D, I suppose.

Cool video showing what life would be like on a hypersphere

This might be a dumb question, but what exactly is the manifold he's on in the video? Like, all points x y z w with x 2 + y 2 + z 2 + w 2 = 1, but with all w < 0 "ground"? Doesn't that make it more of a hemihypersphere? I'm very confused, but it looks really cool.

That is exactly right, half the volume is taken up by the ground in this case. If the camera would be exactly at ground level, it would appear to be flat and you could see the camera itself in all directions along it. As it stands, the furthest you can see is just under half the greatcircle distance away.

Do radio telescope sensors have 2d resolution or do they yield a single signal?

A basic radio antenna indeed only receives a 'single' signal (a voltage fluctuating rapidly as a function of time), which can be processed to determine how much power comes in for the range of spectral components that the antenna and receiver are sensitive to.

But: if you use a suitable dish to focus the radio waves, you can sample the image plane at multiple points (like an optical telescope does, but at a much lower resolution) and make a (coarse) image. You basically use multiple small antennas in the focal plane of the dish, each of which yields its own signal. A nice example of this is APERTIF, on the Westerbork Synthesis Radio Telescope. See https://old.astron.nl/astronomy-group/apertif/apertif.

APERTIF actually uses this system on multiple dishes, so that all those signals can also be correlated with each other across multiple groups of pickup antennas to provide some impressive imaging capabilities at higher resolution, and over a larger fraction of the sky at any given time.

A simulation of how an incoherent light source looks like in slow motion.

This is a great visualisation! It is really nice that you show the kind of information that remains accessible when considering the system at different time scales.

By the look of things you are simulating a monochromatic source, I think. Would it be easy to consider a broadband source of radiation (with frequencies spanning about a factor ten in range)? I suppose the radiation field will look much more complicated and messy (perhaps so much so that it does not tell you much of interest), but who knows - it might show other interesting phenomena.

Hyperbolic maze

This is lovely! I have been playing around with some simulations of hyperbolic geometry too, and I have been thinking about which game mechanisms would be particularly suitable to use in a hyperbolic space. This game makes great use of the fact that you quickly lose your bearings in such a world, as there is, loosely put, much more space located in any given direction than you would expect based on our experience with (more or less) flat space. You can be somewhere where all game locations are actually close by, but you have to know exactly in which direction to go in order to find any particular one of them.

I'm looking forward to this game that Code Parade is planning to release later this year, which seems to involve both negatively curved (hyperbolic) and positively curved (spherical) space.

Ever played HyperRogue? It's really fun, though I'm not sure what part of it is the hyperbolic geometry and what is just the game mechanics that could as well work in euclidean space. Either way there's a free version with basically all the features, and it runs from an .exe directly with no installer, so it's very easy to just try it :)

Yes, absolutely! I think HyperRogue is brilliant as well, although I have never quite managed to complete it. It makes me think about how the game world is stored - I recall vaguely that the author documented quite a few of the implementation details on a dedicated website or blog.

I suppose there are two ways about keeping track of the game world: either you make the game world periodic, like a large-scale repeating 'tile' (like this hyperbolic maze of this post is) and keep just one ɼopy' of it in memory, or you generate chunks of the world on the fly and store the environment of the player out to some critical radius, perhaps with the player-visited areas lingering in memory longer so they can be revisited. Any other way runs into memory problems quite soon, I imagine.

I'm looking for a resource to analyze astronomical images, and could do with some help.

That sounds like an interesting project! While you could search for raw FITS files and extract all the visible structures and stellar images from there, that would be quite a painful process as you would likely need to re-invent the wheel regarding the extraction of useful information from pixel arrays, or otherwise use some difficult specialist tool for that.

A much more convenient way to get astronomical data on at least stellar sources is something like the GAIA data archive, which is freely accessible. It contains data on over a billion stars in our Galaxy: their positions on the sky, their brightnesses, colours, velocities and distances. The database is relatively easy to use with a powerful query language, and tutorials on how to access it and get specific data out are available online at https://gea.esac.esa.int/archive-help/index.html.

A much more convenient way to get astronomical data on at least stellar sources is something like the GAIA data archive, which is freely accessible.

This is something I was also thinking of, and I was half hoping that someone could point me in the right direction. Thanks!

There's only one issue with using this method, and it is a creative one. The interesting thing about FITS data/image data is that it frames a section of the sky. That kind of framing is crucial to creative projects. Would you know whether this kind of constraint can be realized/emulated with the GAIA archive, or something similar?

Absolutely! The archive fully supports selection of specific ranges of sky coordinates. You can define a simple 'rectangle' on the sky (by specifying limits for both right ascension and declination, or expressed in some other coordinate system) or use a more complex shape (such as a specific constellation). It's probably simplest if you browse the linked tutorials and documentation a little to find out how the syntax works (it has been quite a while since I used the archive, and you might as well learn directly from the site).

You can also select stars by volume, distance, speed of proper motion, estimated mass. lots of options!

Physics / astronomy question!

This is also something that a colleague of mine and myself have been working on in our spare time. We have made an interactive app, written in javascript and WebGL, where the user can freely fly around outside a Schwarzschild (i.e., non-rotating) black hole (BH) and see how the sky background gets warped because of the curved spacetime around the black hole (I'm not linking to it yet, because we still want to polish it a bit more before we release it). We use a deflection angle lookup table in a shader to quickly map each camera pixel to the correct sky background pixel. We calculated the deflection angle map by integrating light rays from the camera through the spacetime around the BH for a range of distances to the BH and for a range of look angles w.r.t. the BH. All the light deflections you see in the app are hence correct according to General Relativity.

In order to do ray tracing in curved spacetime, you will need to integrate the geodesic equation for a massless particle (a photon) through the Schwarzschild (or some other) metric. This metric basically describes the structure of spacetime around your black hole. This PDF file provides some useful info for deriving the necessary Christoffel symbols (the capital Greek Gamma symbols that appear in the geodesic equation). With the geodesic equation and the correct expressions for the Christoffel symbols you have all the information needed to do ray tracing in curved spacetime.

The next challenge would be to do this for a rotating black hole - but then the lookup table suddenly gets 4-dimensional and needs a lot more memory. A low-fidelity version should still work, but to make a really nice one will likely take some more trickery to get right.

Why are we looking for Earth-sized planets in the habitable zone? What is unique about Earth's relative size in particular?

On a planet twice the size of earth with the same density as earth, you would weigh twice as much as you do now.

Depends on how you define size: at the ame density but twice the diameter the volume (and mass) would 8 times as much, so youɽ weigh about 8 times as much. To have twice as much gravity as Earth the diameter needs to be only about 25% more than Earth. To have half the gravity the diameter would need to be about 20% less.

Careful there: the mass may be 8 times larger, but your distance from the planet's center of gravity will also be 2 times larger - this reduces the gravitational acceleration by a factor of 4 compared to what it would be using the original planetary radius. So the net effect is that, when we assume a constant density, the surface gravitational acceleration scales with the planet radius.

A Close Look at the Black Hole in the Milky Way

Most galaxies are hosts to Super-Massive Black Holes (SMBHs) lying at their centers. The Milky Way is no different. Every black hole has an event horizon, a region beyond which nothing can emerge, not even light. Therefore direct observations of the black hole itself are currently impossible, and astronomers have to settle for observations of the areas surrounding the black holes.

Such an observation was recently made by an international team of astronomers led by Sheperd Doeleman of MIT. The team was able to detect a structure near the Milky Way’s black hole, at an angular scale of 37 micro arc-seconds. This could be compared to detecting a baseball on the moon. In order to perform this feat researchers had to combine radio wave emissions captured by telescopes in Hawaii, Arizona, and California, using a technique called Very Long Baseline Interferometry (VLBI) . “This technique gives us an unmatched view of the region near the Milky Way’s central black hole,” said Sheperd Doeleman . “No one has seen such a fine-grained view of the galactic center before,” agreed Jonathan Weintroub of the Harvard-Smithsonian Center for Astrophysics, another team member. “We’ve observed nearly to the scale of the black hole event horizon.”

The team studied 1.3 mm radio wave emissions from an object named Sagittarius A*. Emissions at this wavelength are more likely to escape the galactic center, because they are not as susceptible to interstellar scattering which results in a blurred image. The VLBI technique is normally limited to wavelengths of 3.5 mm or longer, but the team was able to push the technique’s limits and obtain results for a shorter wavelength of 1.3 mm.

The structure they saw was located at the galactic center and had a 37 micro arc-second angular scale, corresponding to about 30 million miles. They were able to only vaguely determine its shape. Further research is required to understand what this structure is. Options include a glowing corona surrounding the black hole, an orbiting “hot spot” or a material jet.

“This pioneering paper demonstrates that such observations are feasible,” commented theorist Avi Loeb of Harvard University, who is not a member of the discovery team. “It also opens up a new window for probing the structure of space and time near a black hole and testing Einstein’s theory of gravity.” Furthermore, “This result, which is remarkable in and of itself, also confirms that the 1.3-mm VLBI technique has enormous potential, both for probing the galactic center and for studying other phenomena at similar small scales,” said Weintroub.

The team plans to continue its work by developing novel instrumentation to make more sensitive 1.3-mm observations possible. They also hope to develop additional observing stations to enhance the detail in the picture. Future plans also include observations at shorter, 0.85-mm wavelengths. However, for such work to be successful, they will have to stretch the capabilities of the instrumentation even further and wait for excellent weather conditions at all telescope sites.

TFOT reported on research confirming the leading theory regarding the behavior of galactic black holes, according to which the particles are accelerated by tightly-twisted magnetic fields close to the black hole. In another article TFOT covered a new study suggesting there is an upper limit on the mass of black holes.

Further information on the new discovery, which was published in the Sept. 4 th issue of Nature, can be found in the Harvard-Smithsonian Center for Astrophysics press release.

Something is lurking in the heart of Quasar 3C 279

Illustration of multiwavelength 3C 279 jet structure in April 2017. The observing epochs, arrays, and wavelengths are noted at each panel. Credit: J.Y. Kim (MPIfR), Boston University Blazar Program (VLBA and GMVA), and Event Horizon Telescope Collaboration

One year ago, the Event Horizon Telescope (EHT) Collaboration published the first image of a black hole in the nearby radio galaxy M 87. Now the collaboration has extracted new information from the EHT data on the distant quasar 3C 279: they observed the finest detail ever seen in a jet produced by a supermassive black hole. New analyses, led by Jae-Young Kim from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, enabled the collaboration to trace the jet back to its launch point, close to where violently variable radiation from across the electromagnetic spectrum arises.

The results are published in the coming issue of Astronomy & Astrophysics on April 7th, 2020.

The EHT collaboration continues extracting information from the groundbreaking data collected in its global campaign in April 2017. One target of the observations was a galaxy 5 billion light-years away in the constellation Virgo that scientists classify as a quasar because an ultra-luminous source of energy at its center shines and flickers as gas falls into a giant black hole. The target, 3C 279, contains a black hole about one billion times more massive than our Sun. Twin fire-hose-like jets of plasma erupt from the black hole and disk system at velocities close to the speed of light: a consequence of the enormous forces unleashed as matter descends into the black hole's immense gravity.

To capture the new image, the EHT uses a technique called very long baseline interferometry (VLBI), which synchronizes and links radio dishes around the world. By combining this network to form one huge virtual Earth-size telescope, the EHT is able to resolve objects as small as 20 micro-arcseconds on the sky—the equivalent of someone on Earth identifying an orange on the Moon. Data recorded at all the EHT sites around the world is transported to special supercomputers at MPIfR and at MIT's Haystack Observatory, where they are combined. The combined data set is then carefully calibrated and analyzed by a team of experts, which then enables EHT scientists to produce images with the finest detail possible from the surface of the Earth.

For 3C 279, the EHT can measure features finer than a light-year across, allowing astronomers to follow the jet down to the accretion disk and to see the jet and disk in action. The newly analyzed data show that the normally straight jet has an unexpected twisted shape at its base and, revealing features perpendicular to the jet that could be interpreted as the poles of the accretion disk where the jets are ejected. The fine details in the images change over consecutive days, possibly due to rotation of the accretion disk, and shredding and infall of material, phenomena expected from numerical simulations but never before observed.

Animation showing a zoom into 3C 279 and the jet motions within one week. Credit: Event Horizon Telescope

Jae-Young Kim, researcher at MPIfR and lead author of the paper, is enthusiastic and at the same time puzzled: "We knew that every time you open a new window to the Universe you can find something new. Here, where we expected to find the region where the jet forms by going to the sharpest image possible, we find a kind of perpendicular structure. This is like finding a very different shape by opening the smallest Matryoshka doll."

Avery Broderick, an astrophysicist working at the Perimeter Institute, explains "For 3C 279, the combination of the transformative resolution of the EHT and new computational tools for interpreting its data have proved revelatory. What was a single radio 'core' is now resolved into two independent complexes. And they move—even on scales as small as light-months, the jet in 3C 279 is speeding toward us at more than 99.5% of light speed!"

Because of this rapid motion, the jet in 3C 279 appears to move at about 20 times the speed of light. "This extraordinary optical illusion arises because the material is racing toward us, chasing down the very light it is emitting and making it appear to be moving faster than it is," clarifies Dom Pesce, a postdoctoral fellow at the Center for Astrophysics | Harvard & Smithsonian (CfA). The unexpected geometry suggests the presence of traveling shocks or instabilities in a bent, rotating jet, which might also explain emission at high energies such as gamma-rays.

Anton Zensus, Director at the MPIfR and Chair of the EHT Collaboration Board, stresses the achievement as a global effort: "Last year we could present the first image of the shadow of a black hole. Now we see unexpected changes in the shape of the jet in 3C 279, and we are not done yet. As we told last year: this is just the beginning."

"The EHT array is always improving," explains Shep Doeleman ot the CfA, EHT Founding Director. "These new quasar results demonstrate that the unique EHT capabilities can address a wide range of science questions, which will only grow as we continue to add new telescopes to the array. Our team is now working on a next-generation EHT array that will greatly sharpen the focus on black holes and allow us to make the first black hole movies."

Opportunities to conduct EHT observing campaigns occur once a year in early Northern springtime, but the March/April 2020 campaign had to be cancelled in response to the CoViD-19 global outbreak. In announcing the cancellation Michael Hecht, astronomer from the MIT/Haystack Observatory and EHT Deputy Project Director, concluded that: "We will now devote our full concentration to completion of scientific publications from the 2017 data and dive into the analysis of data obtained with the enhanced EHT array in 2018. We are looking forward to observations with the EHT array expanded to eleven observatories in the spring of 2021."

ALMA Debuts High-Resolution Results

The exciting results of the highest-resolution test campaign yet attempted by the Atacama Large Millimeter/submillimeter Array (ALMA) are detailed in a recent set of four papers.

Animation (click to watch) of the asteroid Juno as seen in mm wavelengths by ALMA’s Long Baseline Campaign. Image credit: ALMA (NRAO/ESO/NAOJ)

ALMA’s array of antennas can be configured so that the baseline of the simulated telescope is as small as 150 m or as large as 15 km across. In its smaller configurations, ALMA studies the large-scale structure of cold objects in the Universe — and this is how the array has been used since it began its first operations in 2011. But now ALMA has begun to test its long-baseline configuration, in which it is able to make its highest-resolution observations and study the small-scale structure of objects in detail.

The Targets

ALMA’s Long Baseline Campaign, run in late 2014, observed five science targets using 22–36 antennas arranged with a baseline of up to the full 15 km. The targets were selected to push the limits of ALMA’s capabilities: each target has a small angular size (less than two arcseconds) with fine-scale structure that is largely unresolved in previous observations. Two of the targets, the variable star Mira and the active galaxy 3C138, were primarily used for calibration and comparisons of ALMA data to those of other telescopes. The remaining three targets not only demonstrated ALMA’s capabilities, but also resulted in new science discoveries.

ALMA’s highest resolution observation yet, of the gravitationally lensed galaxy SDP.81. The maximum resolution of this image is 23 milliarcseconds. Image credit: ALMA (NRAO/ESO/NAOJ) B. Saxton NRAO/AUI/NSF

• Juno is one of the largest asteroids in our solar system’s main asteroid belt. ALMA’s observations of Juno were made when the asteroid was approximately 295 million km from Earth, and the ten images ALMA took have been stitched together into a brief animation that show the asteroid tumbling through space as it orbits the Sun. The resolution of these images — enough to study the shape and even some surface features of the asteroid! — are unprecedented for this wavelength.
• HL Tau is a young star surrounded by a protoplanetary disk. ALMA’s detailed observations of this region revealed remarkable structure within the disk: a series of light and dark concentric rings indicative of planets caught in the act of forming. Studying this system will help us understand how multi-planet solar systems like our own form and evolve.
• The star-forming galaxy SDP.81 — located so far away that the light we see was emitted when the Universe was only 15% of its current age — is gravitationally-lensed into a cosmic arc, due to the convenient placement of a nearby foreground galaxy. The combination of the lucky alignment and ALMA’s high resolution grant us a spectacularly detailed view of this distant galaxy, allowing us to study its actual shape and the motion within it.

The observations from ALMA’s first test of its long baseline demonstrate that ALMA is capable of doing the transformational science it promised. As we gear up for the next cycle of observations, it’s clear that exciting times are ahead!

Citation:

ALMA Partnership et al. 2015 ApJ 808 L1, L2, L3 and L4. Focus on the ALMA Long Baseline Campaign

Fig. 3

The continuum sensitivities, as a function of wavelength, of SOFIA’s mid- to far-infrared instrument suite. Shown are the 4 σ minimum detectable continuum flux densities for point sources in janskys for 900 s of integration time.

2 VLBI Modeling of the Structure of Sgr A*

Traditional VLBI imaging begins by correlating data taken on an array of radio telescopes, yielding complex correlation coefficients for each antenna-antenna baseline for each integration time and bandwidth channel. Each baseline, time, and frequency datum can be placed in a (u,v) plane with axes representing the projected baseline lengths (measured in wavelengths) in the north-south and east-west directions. A single baseline traces out an elliptical path in this plane as the Earth rotates, providing correlation measurements for many baseline lengths and orientations (see Figure 3 for an example). Interferometric imaging hinges on the fact that the (u,v) plane and the sky'' plane are Fourier Transform pairs, so the calibrated correlation data are transformed from the (u,v) plane to make a sky image. Typically, this image must go through a further process of deconvolution to remove the effects of sparse sampling in the (u,v) plane.

Phase and amplitude calibration of VLBI data assume that calibration parameters can be stationized,'' so that the proper calibration for a baseline is the product of two complex gains, one for each antenna in the baseline. This is a valid way to describe the array response under most circumstances. Unfortunately, it can be difficult to obtain calibration measurements with the frequency or the accuracy necessary to produce a high quality image. With this assumption of station-based calibration, however, one can compute two quantities which are independent of the calibration, the closure phase and the closure amplitude. The closure phase is a sum of the interferometric phase around a triangle of antennas (three baselines in a closed loop) all sources of station-based errors cancel and leave a quantity that directly relates to source structure. Similarly, the closure amplitude is calculated by forming the ratio of products of complex correlation data around a closed loop of four antennas (Figure 1).

Figure 1: Closure quantities used in VLBI analysis. Left shows triangle of stations over which closure phase is computed. Baseline 12 is labeled with the measured complex correlation for that antenna pair. On the right is a quadrangle of baselines used to compute a closure amplitude. Expressions for each closure quantity are below each diagram. Closure quantities are independent of calibration of the antennas in the array. (Click here for a PostScript version.)

Calibration is especially difficult for mm VLBI experiments and even more so for mm VLBI experiments targeting Sgr A*. Contributions to errors in the calibration include low sensitivity, variable atmospheric opacity, variable atmospheric path length, antennas deformation under gravity, and antenna pointing errors. These problems are all the more severe for the case of Sgr A*, which is observed only at low elevation from the Northern hemisphere. Thus, it is often preferable to directly model the closure quantities and perform a X 2 minimization to determine the most likely structure given a family of parameterized source models. The relatively simple structure of Sgr A* and the relatively small number of VLBI data points make this approach both computationally tractable and an efficient way to search for structural asymmetries and deviations from the scattering law.

Is there work underway to push the long baseline capabilities of the Event Horizon Telescope to sub-millimeter wavelengths? - Astronomy

On behalf of the US Newton X-ray Multi-Mirror Observatory ( XMM-Newton ) users community, the US Instrument Teams, the NASA/GSFC Guest Observer Facility, and the NASA XMM-Newton Education and Public Outreach program, we request funding for the continued support of US participation in the European Space Agency XMM-Newton mission. The reasons for doing so are compelling: the exceptional scientific return, the complementarity of XMM-Newton and Chandra observations, and the particularly low cost to NASA for enabling access by US astronomers to Great-Observatory class XMM-Newton observations. The response by the US astronomical community to the first two opportunities for submitting XMM-Newton Guest Observer proposals was extensive, with roughly one third of the proposals in both AO's having US PIs. In the first Announcement of Opportunity their success was spectacular, with two thirds of the accepted proposals having either US PIs or Co-Is. ESA does not have the resources to provide support to the large base of XMM-Newton users, and is looking to the US GOF to provide support to the US community. XMM-Newton observations have obtained excellent data for the study of a wide variety of astrophysical phenomena primarily supporting the NASA Structure and Evolution of the Universe theme. The following pages detail the scientific return that XMM-Newton has provided, and will continue to provide, the activities of the US instrument teams, the services provided by the NASA/GSFC GOF, NASA XMM-Newton E/PO activities, and the proposed budget.

XMM-Newton is the second cornerstone in the Horizon 2000 program of the European Space Agency (ESA). XMM-Newton was launched on 1999 December 10, and is in full operation. Over 1500 targets have been observed, with now greater than 70% observing efficiency on average. Over 125 refereed papers have been published. All science data are made public after the expiration of the proprietary period - one year for Guest Observer (GO) data. The XMM-Newton archive opened on 2002 April 15, and there are over 310 observations publicly available as of May 1. For maximum benefit to the science community, ESA is sponsoring the XID program through the Survey Science Centre (SSC). The XID program is designed to identify and perform optical follow-up of serendipitous X-ray sources detected by the EPIC CCD cameras. All XID results will be publicly available through the science archive at the XMM-Newton SOC.

XMM-Newton has lived up to its expectations of high throughput X-ray spectroscopy for the widest variety of astrophysical sources, from comets to quasars. The data obtained in the first two years of operation are just a taste of what is possible to obtain. The enormous archive of serendipitous sources has not yet been fully utilized, and the US GO community is just getting started on the scientific analysis of their data. Like ASCA and ROSAT before it, XMM-Newton has many productive years ahead. The combination of Chandra , Astro-E2 , and XMM-Newton will give the world community the best possible combination of high angular resolution, high throughput, and high spectral resolution. Given the success of the US community in the first AO and the high proposal pressure (roughly eight times oversubscribed), XMM-Newton is clearly perceived as one of the world's pre-eminent astronomical observatories, and will continue to be so for the foreseeable future. Without US community access to XMM-Newton , US scientists will be at a severe disadvantage in many areas of research.

The science goals and achievements of XMM-Newton are directly responsive to the NASA Office of Space Science Strategic Plan. With respect to Table II of the plan http://www.hq.nasa.gov/office/codez/plans/SSE00plan.pdf (page 15), XMM-Newton provides unique or important data for all of the first three elements, and part of the element on solar variability. XMM-Newton allows studies of the fundamental processes of neutron stars and black holes, the creation of the elements in supernova explosions, the dispersal of the elements in supernova remnants and starburst galaxies, the evolution of the elements on the largest scale in clusters and groups of galaxies, and the distribution of dark matter in clusters, groups, and elliptical galaxies. The study of the nature of active stars allows for direct comparison with the early solar system and star forming regions for understanding the origin and evolution of stellar systems. XMM-Newton has determined the positions and spectral characteristics of gamma-ray bursts and examined relativistic processes from neutron stars to quasars. XMM-Newton also provides the unique capabilities of the Optical Monitor (OM) which obtains UV and optical data simultaneously with the X-ray instruments. While there is strong potential overlap in the science areas of Chandra and XMM-Newton , each mission has been optimized differently in the five dimensional space of angular resolution, bandpass, collecting area, spectral resolution, and timing ability. It is thus difficult to precisely define the areas in which one mission is superior or inferior to the other, but the world wide astronomical community has decided that both are important with each mission receiving over 800 proposals per AO cycle.

XMM-Newton was designed to observe astrophysical objects over the keV band. Its large collecting area and highly elliptical orbit allow for long, uninterrupted observations of X-ray sources with an unprecedented sensitivity. The observatory consists of three co-aligned high-throughput 7.5 m-focal-length telescopes with HPD ( FWHM) angular resolution. XMM-Newton provides images over a field of view using the European Photon Imaging Camera (EPIC) detectors, which are CCD arrays (two MOS type and one PN type). High-resolution spectra ( ) are provided by the Reflection Grating Spectrometers (RGS) that deflect half of the beam from two X-ray telescopes (those with the EPIC MOS detectors). The sixth instrument is the Optical Monitor (OM), a co-aligned 30 cm optical/UV telescope sensitive in the 1600-6500 Å band. All scientific instruments operate simultaneously, providing exceptionally rich data sets. All of the detectors can be run in a variety of modes, allowing them to be tuned'' to the desired science needs, angular, spectral, and temporal, of a given observation. Source positions for faint XMM-Newton sources have an RMS uncertainty of over the entire field. The calibration accuracy has reached the 5% level both within and between the EPIC MOS, PN, and RGS, and promises to improve significantly.

During the first 18 months after launch, a variety of software and ground segment problems prevented speedy delivery of data to the GO community. These problems were solved by the fall of 2001, and over 93% of all observations are now delivered within six weeks of observation. The instruments are functioning perfectly except for three anomalies: in the first nine months of operations, two of the 18 chips in the RGS failed and the OM has a stray light problem and a somewhat reduced UV sensitivity. However after these initial difficulties no other lasting problems have occurred. The spacecraft is operating normally, and is more stable than expected before launch. Detailed analysis of the instrument performance shows that the anticipated life of the instruments is consistent with pre-launch estimates of more than 10 years.

Since the launch of XMM-Newton , the relative allocation of time between the Calibration and Performance Verification (Cal/PV), Guaranteed Time (GT), and GO time was heavily modified from the original plan due to the mission operation difficulties. These resulted in much lower than anticipated observing efficiency during the first year of operation. ESA, on the advice of its Science Policy Committee, has decided to complete the GT program by 2003 January. This policy effectively extends the period of AO-1 for the GO community.

The XMM-Newton GO program is fully open to US participation (including US participation in peer reviews). Proposals in response to the AO for the first cycle of open observations (AO-1) were due in 1999 April and those for AO-2 were due in 2001 October. Subsequent AOs will occur annually (with AO-3 due about 2003 March). The peer review of AO-2 has been delayed because of the ESA decision concerning the completion of the GT program, and it is anticipated that the results will be known by mid-July.

European Photon Imaging Cameras (EPIC)

The EPIC instruments provide the X-ray imaging capabilities for the XMM-Newton observatory, and are complimentary to Chandra's imagers with significantly larger effective area and field of view but lower angular resolution. The EPIC MOS and PN instruments are functioning nominally, with angular and energy resolution as designed. In periods of low background, the background rates are consistent with preflight estimates. As with Chandra , there are intervals when the background rate increases significantly due to soft protons. Roughly 10% of the total time is contaminated by such flares, similar to Chandra .

All EPIC operating modes have been verified, including the burst and timing modes of the PN and the windowing modes of the MOS and PN. The degradation of the energy resolution by solar particles (the increase in the CCD charge transfer inefficiency, CTI) is as expected before launch, and the instrument should have at least a ten-year lifetime.

 Total Energy Band keV Field of View diameter PSF FWHM, HPD Timing Resolution 7 s to 2.5 s, mode dependent Spectral Resolution 55 eV at 1 keV Sensitivity ergs cm s Effective Area 2484 cm at 1.5 keV

Reflection Grating Spectrometers (RGS)

Two RGS units provide high-throughput, high-resolution spectroscopy of point-like and moderately extended sources. Both RGS instruments are operating nominally with the exception of one CCD on each RGS. However, the lost CCDs cover different wavelength bands so the loss of science capability is minimal.

The spectroscopic resolving power ranges from 200 to 800 over the keV band, and is intermediate between the HETG and LETG spectrometers on Chandra , but with significantly higher effective area (counting rates are typically higher). With its exceptionally large dispersion angles, the RGS has a unique capability for spectroscopy of moderately extended sources (full spectral resolution for source diameters up to , and useful resolution for sources up to in radius), which has been applied with great success.

As with EPIC, the CTI of the RGS CCDs is slowly increasing over time. The increase is at the levels that were budgeted in the design, indicating that the RGS CCDs should remain functional throughout the planned ten-year lifetime of the mission.

 Total Energy Band Å ( keV) Field of View (in cross-dispersion) Timing Resolution 16 ms to 9 s, mode dependent Spectral Resolution 0.06 Å (-1), 0.04 Å (-2) Sensitivity ergs cm s Effective Area 180 cm at 15 Å (-1, -2 comb.)

The OM is a 30 cm Ritchey-Chretien optical/UV telescope co-aligned with the X-ray telescopes, and can provide simultaneous optical/UV imaging, spectroscopy, and photometry with the X-ray data. This is a capability not matched by any other observatory. The microchannel-plate intensified photon counting CCD detector allows precision timing with a resolution of 0.5 s (with work underway for 0.05 s resolution). The OM has a wide variety of modes, but basically covers a field of view, sensitive in the 1600-6500 Å band, with an angular resolution of . The sensitivity in the 3000-6500 Å band is as expected but there has been a reduction in the UV sensitivity compared to pre-launch calculations. There has been no change in instrument performance with time.

 Total Bandwidth Å (6 bands) Spectral Bandwidth Å (two grisms) Sensitivity Limit V = 23.5 m in 1 ks Field of View PSF (FWHM) (filter dependent) Timing Resolution 0.5 s (will be 0.05 s) Spectral Resolution 5.0/10.0 Å (two grisms) Spatial Resolution /pixel Brightness Limit V 7.4 m (filter dependent)

Because of the wealth of XMM-Newton data and results, we have focused on a small number of what we believe to be the more interesting results. We have tried to represent some of the broad science that XMM-Newton has performed, and is capable of, but realize that this has led us to be quite selective. Given that there have been a total of more than 1700 XMM-Newton GO proposals, the breadth and depth of XMM-Newton science is representative of a Great Observatory'' like Chandra or Hubble . Some of the breadth of XMM-Newton can be seen in the abstracts for the 2001 November ESTEC meeting New Visions of the X-ray Universe in the XMM-Newton and Chandra era'' http://www.estec.esa.nl/conferences/01C12/, the special issue of Astronomy and Astrophysics devoted to XMM-Newton results (Volume 365), and the GOF publication list http://heasarc.gsfc.nasa.gov/docs/xmm/xmmhp_bibliography.html

1.1 Active Galactic Nuclei

A large fraction of the XMM-Newton observing time has gone to this field, and the early results are commensurate with the observation time with over 35 published refereed papers. We have focused on four fundamentally new discoveries.

Since the early ASCA discoveries of relativistically broadened Fe K lines, there have remained lingering detailed problems of interpretation. XMM-Newton has made a major contribution in this area. In the case of MCG-6-30-15, Wilms et al. (2002) have shown that the line is so broad that much of the region producing it must lie extremely close to the central object, requiring the black hole to be spinning rapidly. The intensity and shape of the line are inconsistent with an origin in an externally illuminated accretion disk and, the authors suggest that a large fraction of the energy comes from direct extraction from the black hole rather than accretion.

Figure 1: The lower panel shows the 2-10 keV data on NGC 4051 taken with the PN (curve) and RXTE (diamonds, normalized to the PN data). The upper panel shows the OM UV data (solid circles) with the reprocessing model (Mason et al. 2002).

Despite the extensive optical and X-ray data previously obtained on AGN, the connection between these two bands is not well understood. The reflection continuum'' present in many Seyfert 1 galaxies strongly suggests that 50% of the total X-ray energy is reprocessed into UV and optical light. However, in the most detailed study to date, Nandra et al. (1999) showed that there was no correlation between X-ray and UV light curves in the RXTE and IUE observation of NGC 7469. In a 1.5 day XMM-Newton observation of NGC 4051 (a low luminosity Seyfert I galaxy), Mason et al. (2002) report that the UV lags the X-ray by d (much longer than expected for an accretion disk), and that % of the total UV flux originates in the reprocessed component (Figure 1). A geometric model of the reprocessing region suggests that it is ringlike, covers less than 20% of the central source, is light days from the central object, and is inclined to the line of sight by degrees. The inability to detect this correlation in previous studies was probably due to insufficient sampling and too short a baseline.

Figure 2: RGS1 (red) and RGS2 (blue) spectra of NGC 1068. Line labeling indicates the final state ion. All H-like ( ) and He-like (r, i, and f) principal order lines are labeled (Kinkhabwala et al. 2002).

The spectacular XMM-Newton RGS spectrum of NGC 1068 (Figure 2) shows strong, narrow radiative recombination continua, implying that most of the soft photons arise in low-temperature plasma (kT few eV). This plasma is photoionized by an inferred nuclear continuum (obscured along our line of sight). Compared to pure recombination, there is excess emission in all resonance lines up to the photoelectric edge, demonstrating the importance of photoexcitation as well. A cone of plasma irradiated by the nuclear continuum provides a remarkably good fit to the H-like and He-like ionic line series, with inferred radial ionic column densities consistent with recent observations of warm absorbers in Seyfert 1 galaxies. These data and the Chandra images imply that the warm absorber in NGC 1068 is a large-scale outflow. To explain the ionic column densities, a broad distribution of the ionization parameter is necessary, spanning log ergs cm s . This suggests either radially stratified ionization zones or the existence of a broad density distribution (spanning a few orders of magnitude) at each radius.

The XMM-Newton RGS detected for the first time (in IRAS 13349+2438) a broad absorption feature around 16-17Å identified as an unresolved transition array of 2p-3d inner-shell absorption by Fe (Sako et al. 2001). The M-shell ions originate in a much cooler medium than the other X-ray absorption features, implying the existence of at least two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

Because of their low flux and low surface brightness, spatially resolved spectroscopy of clusters at greater than one third of the virial radius, and groups over their entire extent, was difficult with Beppo-SAX and ASCA . The important scientific problems of the mass distributions, chemical composition, and cooling flows require XMM-Newton 's capabilities.

Cooling Flows: One of the most surprising results from XMM-Newton has come by combining RGS spectroscopy of cooling flows with EPIC spectroscopic imaging. While the distribution of projected temperature with radius derived from the EPIC data is in excellent agreement with the deprojection analysis of ROSAT and ASCA data, it is in strong contradiction to standard cooling flow theory. The gas is not multi-phase and shows much less cool material than expected.

The total soft X-ray luminosity is roughly consistent with the predicted deposition rate, but the emission measure distribution is considerably steeper than the standard cooling flow model (Peterson et al. 2002). The expected emission lines of O VII , Fe XVII , and Fe XX from an isobaric cooling flow model are simply not present, but Fe XXII , Fe XXIV , and other low-energy lines from gas at 50% of are observed at the predicted level. Similar results are obtained from elliptical galaxies (Xu et al. 2002).

It should take six times longer for gas in the hot phase to cool from 8 to 3 keV than to cool from 3 to 0.1 keV, so it is perplexing why there is no evidence for further cooling after it has cooled by 85% of . It is difficult to find a dynamical timescale, closely connected to the cooling time, which could vary by orders of magnitude depending on the local plasma conditions.

Virial masses of clusters: Before the launch of XMM-Newton , there was considerable controversy about the temperature profiles in clusters (Markevitch et al. 1998, De Grandi & Molendi 2001 Ezawa et al. 1997 White 2000). Some analysis indicated a universal temperature gradient while others indicated isothermal gas. Based on cold dark matter and hydrodynamic simulations, theory predicted a factor of two drop in temperature from the center to 50% of the virial radius (Frenk et al. 1999). It is therefore surprising that the XMM-Newton temperature profiles for a set of clusters (e.g., Pratt et al. 2002) are very flat out to R . This is likely to revolutionize our understanding of the formation of large scale structure, and is a challenge to the standard cold dark matter models. The data allow a determination of the total gas mass and its relative fraction with radius. Under the assumption that clusters are fair samples of the universe, this places an upper limit on the density of the universe at . Furthermore, these new results confirm the scaling between temperature and cluster mass found by ASCA , which lies below the theoretical predictions.

Figure 3: The azimuthally averaged EPIC results for the temperature, O, Si, and Fe abundances in the NGC 4325 group (Mushotzky et al. 2002).

Chemical Abundances: XMM-Newton allows a derivation of the chemical abundances of O, Si, S, and Fe with radius (e.g., Tamura et al. 2001) in a reasonable number of clusters. At large radii the abundances are constant with radius, but in the central kpc there is often a sharp rise in Fe, but not in O. The determination of the O abundances for a fair sample of clusters (Kaastra et al. 2002) provides the first robust test of the origin of the elements. A predominance of Type II SN products exist in the outer regions, with a strong contribution from Type Ia's in the central cusp. The first measurement of abundances in groups (see Figure 3) shows a result similar to that of rich clusters - a flat abundance profile and small changes in the relative abundances with radius.

High Redshift Systems: XMM-Newton is ideal for serendipitous searches for clusters (see Figure 4). Pointings have already resulted in the detection of clusters at , and the detailed study of at least two objects at (Hashimoto et al. 2002). The detection of hot systems at such large redshifts is a strong constraint on all theories of structure formation and an indication of a low density universe.

Figure 4: Mosaic in true X-ray colors of a large solid-angle survey. Red: soft sources ( keV), blue: hard sources ( keV). The individual images have a diameter of (Pierre et al. 2002).

XMM-Newton has obtained fundamentally new information on X-ray binaries in nearby galaxies, hot gas in elliptical galaxies, starburst galaxies, and the nature of diffuse emission in nearby spiral galaxies and the Milky Way.

Rapidly Star forming Galaxies: Starburst galaxies and their superwinds are important contributors to the energizing and metal enrichment of the intergalactic medium (IGM). M82 is one of the nearest and brightest of these. The RGS data have shown a wealth of strong emission lines in the spectrum, originating from a wide range of species and temperatures. These data allow a robust determination of the chemical composition of the central regions, and support a direct search for the signature of production of the elements. The EPIC observations have shown that the superwind extends out to kpc above the disk of the galaxy. These observations can constrain the mass and chemical composition of the superwind, allowing a direct test for the ejection of metals into the IGM. Only XMM-Newton has the combination of angular resolution and collecting area to derive the spectrum of the starburst wind far from the galaxy. Early results on the RGS data for NGC 253 show that the gas is radiating via collisions (Pietsch et al. 2001). The strong Fe XVII lines seen in the nuclear region indicates the presence of a large emission measure of hot gas. If this gas is due to the sum of young supernova remnants, then the implied SN rate is yr .

The nature of X-ray emission from ultra-luminous IR galaxies was the subject of much debate in the ASCA and ROSAT era, and has strong implications for the origin of the IR and X-ray backgrounds. The XMM-Newton data (Braito et al. 2002) indicate that these objects are most often composites, with all of them showing hard emission from a power law component. However in only 60% of the sources is it clear that this component is an active galaxy nuclei. All of them show soft emission from hot gas produced by star formation. These preliminary results agree with Chandra observations of far-IR SCUBA sources and ISO observations of XMM-Newton hard X-ray sources (Franceschini et al. 2002), which show that % of the objects are AGN dominated based solely on their X-ray luminosity. Similar results on three other objects have been obtained by Sanders et al. (2002). The confirmation from XMM-Newton observations that the composite nature of IR selected luminous galaxies extends up to the highest IR luminosities has strong implications for the nature of the highest redshift objects, and the creation of the first black holes.

Nearby Normal Spiral Galaxies: The XMM-Newton data for the M31 X-ray binaries have better spectral resolution and similar sensitivity as UHURU observations of binaries in the Milky Way! For the first time phenomena like bursts, dips, flares, atoll, and Z sources can be studied in other galaxies (e.g., Trudolyubov et al. 2002 Barnard et al. 2002).

Figure 5: Left : RGS spectrum of N103B (van der Heyden et al. 2002). Right : RGS spectrum of 1E0102.2-7219 (Rasmussen et al. 2001).

XMM-Newton has measured the spectral parameters for large samples of ultra-luminous X-ray sources in nearby galaxies (Soria & Kong 2002) and, combined with Chandra observations, have developed the first large body of long-term light curves. Some of these objects are transient in nature. The simultaneous OM UV images provide tight upper limits on possible massive star companions.

In several nearby spirals, XMM-Newton has detected diffuse gas in the nuclear regions. This gas is line dominated and, in the case of M31, has a temperature consistent with the virial temperature of the bulge (Shirey et al. 2001). In M81 the gas is multi-phase, with the bulk of the emission measure at kT keV. These gas temperatures indicate that the gas is in equilibrium with the central potential well in these galaxies.

Elliptical galaxies: The ISM in elliptical galaxies is a reservoir of stellar mass loss and supernovae. The gas is hot and in equilibrium with the dark matter potential well. ASCA and ROSAT data could not uniquely determine the temperature or abundances of the gas because of the inherent degeneracies in low resolution data. These limitations have been overcome by XMM-Newton (Xu et al. 2002). The detection of optically thick Fe XVII lines allowed a direct check of the thermal and spatial models. The derived abundances of O, Mg, Ne, N, and Fe showed sub-solar abundances for all the elements, and solar or sub-solar ratios of [O-Mg/Fe]. This is in sharp contrast to what is expected from the gas being enriched by Type Ia supernovae and stellar mass loss, and presents a strong challenge to the theories of the origin and evolution of elliptical galaxies and Type Ia supernova rates. The RGS data provide the most accurate abundance determination to date for both the stars and the gas in elliptical galaxies.

1.4 SNRs, Neutron Stars, and PNe

As extended objects with high temperatures and a complex set of abundances and ionization conditions, supernova remnants (SNRs) are prime objects for studies with XMM-Newton . Spatially resolved SNR spectra can identify stratification of ejecta, separate the forward and reverse shock regions, measure spatial variations in temperature and ionization conditions, and search for compact hard emission associated with a pulsar or its nebula. The XMM-Newton RGS provides high-resolution spectroscopy of SNRs with radii as large as a couple arc minutes. This permits the use of line diagnostics for density and ionization state determinations, as well as providing sufficient spectral resolution to measure velocities.

Plasma Diagnostics: Plasma diagnostics are at the heart of SNR evolution studies. They are the keys to temperature, ionization, and elemental abundance measurements, leading the way to a better understanding of the progenitor star, a more accurate measure of all the different elements produced in the SN explosion, as well as their relative mixing and ionization states. All these studies require the XMM-Newton capabilities of large collecting area and high spectral resolution.

The results are spectacular. Figure 5 shows the RGS spectra of two young SNRs, N103B and 1E0102.2-7219. N103B displays a rich spectrum indicative of a multi-phase plasma whereas non-equilibrium ionization effects dominate the 1E0102.2-7219 spectrum. Elemental abundances are tracers of different periods in the history of the SNR's evolution: The Fe L lines that are detected in 1E0102.2-7219 most likely trace the swept-up mass, and the absence of N suggests that what was present in the progenitor was either burned into other elements or blown off before the SN event.

Figure 6: EPIC-MOS images of N132D in narrow wavelength bands. Each image is labeled with the principal line-emitting ion in its band (Behar et al. 2001).

The narrow-band EPIC images of N132D (Figure 6) map the elemental and temperature structure in the remnant. With the exception of O, the dominant part of the soft X-ray emission originates from shocked ISM along the southeastern and northwestern edges of the expanding shell. In contrast, strong Fe-K emission is detected near the center, perhaps indicating that the high-Z ejecta are too highly ionized to be observed at longer wavelengths. O is present throughout the remnant, while the brightest spot of O emission is near the northeastern rim. The O emission can be attributed either to low temperatures or to hot, recently shocked material that is in the process of being ionized. On the other hand, the RGS spectrum shows that the intercombination line is very weak whereas the recombination and forbidden lines are strong. This confirms the hot ionizing conditions, and suggests that the O-rich gas is still in the process of being ionized.

XMM-Newton has provided impressive images of SNRs which can be correlated with images at other wavelengths, and used to rule out some production mechanisms. Such a study was done for Cas A (Bleeker et al. 2001) and RCW 86 (Figure 7). In both cases, the lack of correlation between the high energy XMM-Newton and the radio images suggests that the hard X-rays detected in both SNRs are not from synchrotron radiation but rather from non-thermal bremsstrahlung generated by a population of supra-thermal electrons.

Figure 7: Mosaic of exposure corrected EPIC PN and MOS images covering the SW, SE and NW of RCW 86. The image is color coded by the X-ray energy: red - soft, green - medium, blue - hard (Vink et al. 2002).

Composite and Plerionic SNRs: Particles injected from a central pulsar diffuse through the nebula, the synchrotron cooling of the particle population results in a spectrum that should soften with radius. This variation in the power law index with radius was confirmed by XMM-Newton for the Crab Nebula (Willingale et al. 2001) and other Crab-like SNRs (Warwick et al. 2001 for G21.5-0.9 Bocchino & Bykov 2001 for IC443). For both G21.5-0.9 and IC443, images reveal a faint shell extending beyond the radio plerion. The emission is non-thermal in G21.5-0.9 but could not be uniquely characterized in the case of IC443. These X-ray halos are perplexing because the long synchrotron lifetimes of the radio-emitting particles should typically result in larger radio nebulae.

For the first time, XMM-Newton observations show indications of thermal emission from the outer region of the plerion, 3C58, providing the first look at the effects of its blast wave (Bocchino et al. 2001). This result provides crucial information on very young pulsar-driven systems. Measurements carried out with the RGS (van der Heyden et al. 2001) have also revealed such a thermal shell in SNR 0540-69, housing a bright, fast pulsar. Emission from O, Ne, and Fe associated with the shell of 0540-69 reveals a shock velocity in excess of . Such emission can give compelling constraints on the age of the remnant, the shock velocity, and the density of the environment.

XMM-Newton EPIC is proving useful in studies of Galactic diffuse hot plasma sources. Guerrero et al. (2002) observed the planetary nebula (PNe) NGC 7009, and have detected extended X-ray emission in its central cavity. The diffuse X-ray emission originates from the shocked fast stellar wind, and the spectra show that the temperature of the hot plasma is K. The same authors also detect 10 K emission from the WR wind-blown bubble S308.

Gamma-ray bursts: XMM-Newton's fast response time to ToO's has so far resulted in three rapid observations of Gamma-Ray Burst afterglows. Data acquisition was started within 12 hours of the GRB alert, and the data were made available to the community within a very short period. XMM-Newton is in many ways the ideal observatory for the study of GRB X-ray afterglow emission: it provides a large throughput and bandwidth (including optical/UV!), combined with high spectral resolution with no instrumental tradeoffs required. This combination has already yielded one potentially very interesting result. The afterglow spectrum of GRB011211 observed in the PN camera, for a brief 5000 s period at the start of the ToO observation, appears to show discrete emission from the mid-Z elements (Mg, Si, S, Ar, Ca), but not from Fe (Reeves et al. 2002). This naturally suggests the abundance pattern expected from nucleosynthesis in massive stars, which would be the first direct connection between GRBs and massive stellar evolution.

X-ray Binaries: X-ray binaries (XRBs) and cataclysmic variables (CVs) comprise the largest class of galactic X-ray sources. The deposition of ballistic material onto the surface of a degenerate star results in copious X-ray emission, either from the heated atmosphere of the star, or shocks above its surface. Their proximity, well-defined binary geometry, and range of emission (extending over the entire electromagnetic spectrum) result in a class of objects where the fields of accretion, magnetic activity, degenerate plasma physics, and binary evolution meet.

An unexpected result was the detection of Fe K resonance absorption lines in three high-inclination XRBs, suggesting these may be common characteristics of accreting systems with close to edge-on disks (Parmar et al. 2002). The features are identified with the K absorption lines of Fe XXV and Fe XXVI , and since they are detected over a wide range of orbital phases, the absorbing material is most likely located in a cylindrical geometry with an axis perpendicular to the accretion disk. The material responsible is constrained to be close to the central source, at a radius 200 times smaller than the outer edge of the accretion disk. Equivalent widths increase during dips, corresponding to a column of Fe atoms cm . Until recently the only XRBs known to exhibit narrow X-ray absorption lines were superluminal jet sources, and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized Fe absorption features may be common characteristics of accreting systems with accretion disks.

A broad, skewed Fe K emission line has been detected in the transient source X1650 50 (Miller et al. 2002), which suggests that this system hosts a black hole. The line shape indicates a steep disk emissivity profile that is hard to explain in terms of a standard accretion disk model. Similar to the XMM-Newton data for the Seyfert galaxy MCG 6-30-15, these results can be explained by the extraction and dissipation of rotational energy from a black hole via magnetic connection to the inner accretion disk. If this process is at work in both sources, a fundamental prediction of general relativity will be confirmed across a factor of in black hole mass.

Cataclysmic Variables: Despite their ubiquitous nature, CVs have low luminosities ( ergss ), and so detailed X-ray experiments have only been possible on a small, high luminosity sub-sample of the class. With its high-resolution, high-throughput cameras, its ability to observe bright sources in full timing and spectral modes, and its simultaneous optical and UV capabilities, XMM-Newton is revolutionizing CV astronomy.

A long-standing prediction suggests that post-shock flows are stratified in temperature and density. The RGS data place demanding tests on this model Figure 8 compares the best stratified model fit with single temperature zone fits. The remaining small residuals in the lower panel are the result of irradiation of the accretion column. Two well-constrained products of the fit are the mass of the white dwarf and the accretion rate. Both are fundamental CV properties that have proved notoriously difficult to measure in the past. A sample of these quantities is necessary to understand the interdependence of CV phenomena, determine the origins of the systems and predict their future evolution. For example, by comparing magnetic field strengths for CVs and single white dwarfs it is apparent that accretion suppresses fields. Masses and accretion rates will help us understand this process. Extending this analysis, Ramsay et al. (2001) have shown that thermal emission from the white dwarf surface is not a standard feature of the CV spectrum. They argue that only the bright, high mass transfer systems radiate thermal spectra in the X-ray band.

Figure 8: The RGS spectrum of the magnetic CV EX Hya with stratified model fit. Residuals are shown in the bottom panel. Residuals from 1- and 3-temperature MEKAL model fits are shown in the middle panels (Cropper et al. 2002).

Optical and UV emission collected by the OM independently probes the cyclotron emission and the colder atmospheres of the pre-shock accretion flow and companion star. Wheatley & West (2002) have shown that the X-ray and cyclotron emission regions occupy different volumes, the cyclotron region being larger. After detecting a unique new type of X-ray flare from UZ For in its quiescent state, Still & Mukai (2001) and Pandel & Cordova (2002) concluded this was an accretion, rather than coronal, event. Both results would have been impossible without simultaneous OM data.

ASCA observations indicated that Ne was overabundant in many coronal sources, but line blends did not allow conclusive measurements. By taking advantage of large effective area and dispersive spectroscopy, XMM-Newton has settled the argument over the so-called Inverse FIP'' (First Ionization Potential) effect (Brinkman et al. 2001). RGS data on the RS CVn system HR 1099 showed an increase of coronal abundances with their respective FIP (elements with a higher FIP are more abundant), the exact opposite of what is observed in the Sun . This result has been confirmed in the rapid rotator AB Dor (Guedel et al. 2001), both showing anomalously strong Ne IX lines. Audard et al. (2001) find an inverse FIP affect in HR 1099 during quiescence but a direct FIP effect during flares. These flares are thought to be directly associated with the process of element fractionation in stellar atmospheres. Since elements with different FIPs will be ionized by differing amounts and low FIP material has a higher probability of being accelerated along magnetic loops and diffused into the chromosphere, there is a higher abundance of high FIP particles in the lower atmosphere that will be lifted into the corona by flares.

An XMM-Newton RGS observation of the Wolf-Rayet (WR) star WR110, which has no known companion, shows an unexpected and unexplained hard X-ray component (equivalent to a 3 keV thermal plasma). The analysis (Skinner et al. 2002) shows that half of the flux from the star arises from the hard component. Simultaneous VLA observations probed the atmosphere and exclude the possibility that this hard X-ray emission arises in wind shocks, leaving unresolved the mystery of its origin.

Observations of the symbiotic star, Z And, obtained shortly after outburst, show a complex spectrum with photon energies out to at least 5 keV (Sokoloski et al. 2002). Using simultaneous FUSE spectra to fix the absorption column yields a spectrum consisting of an 80 eV blackbody, a 0.7 keV thermal plasma, and a hard tail possibly arising in shocked winds. A follow-up observation obtained five months later will reveal the evolution of the X-ray spectrum during the optical decline.

The diffuse X-ray background is the sum of emission from several different regions of hot plasma: the local ISM, Galactic halo, Local Group intercluster medium (ICM), and the hot IGM. It also includes the previously unresolved extragalactic background now known to arise mostly from AGN (e.g., Hasinger et al. 1998).

Studies of this diffuse emission require large effective areas and large solid angles to provide sufficient counts, low detector background, and good angular resolution to allow removal of contaminating point sources. Not only does XMM-Newton EPIC satisfy all these requirements, but its spectral response is good down to keV, covering the energies where the Galactic halo and the hot IGM are most easily observed.

A major outstanding issue is the origin of the diffuse background at high Galactic latitudes in the keV band. Only about 50% of the observed flux can be attributed to recently resolved cosmological sources. Another 10-20% of the observed flux can be attributed to Galactic emission from thermal plasmas (and a small amount to unresolved stellar contributions). Determining the origin of the remaining fraction has both cosmological and Galactic implications.

Recently, major constraints on the properties of these hot plasmas were determined by absorption studies. Significant O VII and O VIII resonance line absorption at zero redshift were found in deep RGS exposures of the AGN 3C 273, Mkn 421, and PKS 2155-304 (Figure 9). These features, consistent with the detection of O VII and O VIII in the Chandra LETGS spectrum of PKS2155-304 (Nicastro et al. 2002), are definitely at zero redshift. The lines have turbulent Doppler velocities of a few hundred km s or less. Measured column densities in O VII and O VIII are of order a few times ions cm .

Figure 9: Spectra of Capella, 3C273, Mkn 421, and PKS2155-304. Absorption is seen at 18.97 Å (O VIII Ly ) and 21.6 Å (O VII resonance line) in the QSO spectra. Absorption at 23.5 Å is due to neutral O in the Galaxy. The Capella spectrum demonstrates the accuracy of the wavelength scale (Rasmussen et al. 2002)

The density and size of the hot plasma can be established by combining absorption measurement with constraints on emission from the same medium. Since the medium cannot overproduce the keV background, most of the absorption cannot be due to hot plasma in the ISM of our own Galaxy thus we are seeing absorption by hot plasma located in an extended halo of the Galaxy or in the intercluster gas of the Local Group. The electron temperature of this medium, determined from the ratio of column densities in O VII and O VIII , is 200-300 eV, consistent with expectations based on the dynamics of the Local Group galaxies. This opens up a whole new field of study, with measurements constraining the total baryon census at redshift zero, as well as the evolution of the diffuse gas in collapsed structures of low to moderate overdensity.

Figure 10: Left Panel: The distribution of 2-9 keV X-ray emission in the Radio Arc Region. The image covers a region . The bright extended source associated with Sgr A East is visible on the right-hand side of the image. Right Panel: The corresponding hardness (H-M/H+M) ratio distribution (Hard: 5-9 keV, Medium: 2-5 keV, Warwick 2002).

Figure 10 shows the EPIC image and hardness ratio of the 0.4-9 keV emission from the central pc of our Galaxy (Warwick 2002). There is complex structure with large-scale extended X-ray emission associated with the non-thermal radio source Sgr A East, asymmetrically distributed around the region. The diffuse X-ray component present in the Radio Arc Region exhibits considerable spectral variations on scales down to , consistent with a complex mix of thermal and non-thermal contributions. While Chandra has resolved extensive emission in this region into the contributions of point sources (Wang et al. 2002), the origin and nature of the diffuse X-ray emission seen in the Galactic Center Region and in the Galactic Plane in the form of the Galactic X-ray Ridge remain topics of active debate.

12pt 1.8 Cosmology, Surveys, and Serendipitous Science

The ability of XMM-Newton to rapidly survey large fields to faint levels with accurate positions and a reasonable number of X-ray spectra is a breakthrough in this field. Most exposures reach as faint as the deepest ROSAT surveys in the soft band and ten times fainter than ASCA and Beppo-SAX in the hard band. XMM-Newton detects normal galaxies to z , Seyfert galaxies and groups to z , and quasars and rich clusters to arbitrarily high redshift. XMM-Newton data provide a unique database for studying the correlation function of X-ray sources and their clustering properties. XMM-Newton has already undertaken a total of observations at high Galactic latitude with exposure times greater than 50 ks. On average, each such field contains about 50 serendipitous sources of which are bright enough for X-ray spectroscopy, and reaches a flux threshold of ergs cm s in the 0.5-2 keV band (Lumb et al. 2001).

This serendipitous database will provide the largest sample of distant clusters, and allow the measurement of the cosmic evolution of the temperature and luminosity functions of clusters and groups. The XMM-Newton database will be vastly superior to the previous databases used for this purpose. It will push these measurements to higher redshifts, where the sensitivity to the cosmological parameters is greatly enhanced (Henry 2000).

A comparison between the 1 Ms Chandra observation of the Hubble Deep Field North, and a 155 ks XMM-Newton observation of the same field shows that XMM-Newton does extremely well in a much shorter exposure time (Figure 11). There is some source confusion present in the XMM-Newton image, limiting the effective flux threshold in the fields to 2 imes10^<-16>\$ --> ergs cm s , but the confusion limit in the 5-10 keV band has not yet been reached. The two Chandra deep surveys show remarkable differences due to cosmic variance and it is clear that the much larger XMM-Newton solid angle and similar sensitivity will allow it to make a major contribution in this area.

Figure 11: XMM-Newton observation of the Hubble Deep Field North. Red: 0.5-2.0 keV, Green: 2.0-4.5 keV, Blue: 4.5-10.0 keV (Griffiths et al. 2002).

The XMM-Newton SSC XID program
http://xmmssc-www.star.le.ac.uk/ has released multi-color CCD images for many XMM-Newton fields with details of the identifications of sources in the fields, links to optical finding charts, optical photometry of potential counterparts, and reduced optical spectra. This public release of optical follow-up data is unique to XMM-Newton and will continue as part of the SSC function.

The 2-10 keV background has been almost fully resolved by Chandra into point sources. The good response of XMM-Newton at high energies has been used to show that Type 2 Seyferts and unidentified sources are responsible for the bulk of the extragalactic background (Griffiths et al. 2002). The spectra will provide a strong test of the unified models of the X-ray background. What we still need to understand is whether the rates of evolution of sources with different absorption columns are the same or whether they differ. With its large database of X-ray spectra of serendipitous sources, XMM-Newton will make a major contribution in this area.

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Appendix 2: List of Acronyms

 AAPT American Association of Physics Teachers AAS American Astronomical Society AGN Active Galactic Nuclei AO Announcement of Opportunity ASCA Advanced Satellite for Cosmology and Astrophysics Cal/PV Calibration and Performance Verification CCD Charge Coupled Device CDM Cold Dark Matter Co-I Co-Investigator CTI Charge Transfer Inefficiency CU Columbia University CV Cataclysmic Variable DPU Digital Processing Unit EPIC European Photon Imaging Camera E/PO Education and Public Outreach ESA European Space Agency FIP First Ionization Potential FTE Full Time Equivalent FUSE Far Ultraviolet Spectroscopic Explorer FWHM Full Width at Half Maximum FY Fiscal Year GLAST Gamma-ray Large Area Space Telescope GO Guest Observer GOF NASA/GSFC Guest Observer Facility GRB Gamma-Ray Burst GSFC Goddard Space Flight Center GT Guaranteed Time HEASARC High Energy Astrophysics Science Archive Research Center HETG High Energy Transmission Grating HPD Half Power Diameter HTR High Time Resolution ICM Intercluster Medium IGM Intergalactic Medium INTEGRAL International Gamma-Ray Astrophysics Laboratory IR Infra Red IRAS Infra Red Astronomical Satellite ISM Interstellar Medium ISO Infrared Space Observatory LANL Los Alamos National Laboratory LETG Low Energy Transmission Grating MOS MOS style EPIC CCD detector NASA National Aeronautics and Space Administration NSTA National Science Teachers Association OGIP Office of Guest Investigator Programs OM Optical Monitor PI Principal Investigator PPS Pipeline Processing Subsystem PSF Point Spread Function PN PN style EPIC CCD detector PNe Planetary Nebula PPS Pipeline Processing System PV Performance Validation RGA Reflection Grating Array RGS Reflection Grating Spectrometer RMS Root Mean Square ROSAT Röntgen Satellite RPS Remote Proposal System RXTE Rossi X-ray Time Explorer

 SAS Science Analysis System SEU Structure and Evolution of the Universe SEUEF Structure and Evolution of the Universe Education Forum SN Supernova SNR Supernova Remnant SOC Science Operations Center SRON Space Research Organization Netherlands SSC Survey Science Centre SSU Sonoma State University SWT Science Working Team ToO Target of Opportunity UCSB University of California, Santa Barbara URL Universal Resource Locator UV Ultra Violet WR Wolf-Rayet XID X-ray source IDenfication program XMM X-ray Multi-Mirror Mission XRB X-ray Binary

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