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X-Ray Background, everpresent noise

X-ray Background Mystery Resolved
In high energy astronomy, every photon counts, and background events have been as big a mystery as the objects in the foreground. In x-ray astronomy in particular, the unexplained 'noise' takes the form of an apparently diffuse and fairly significant background wherever the detector points.

One component emanates uniformly from all directions in the sky, while another correlates strongly with the disk of the Milky Way galaxy, so we're likely seeing two distinct phenomena and have at least two mysteries to solve. Recently, astronomers have the use of a new tool in the form of the Chandra X-ray Observatory, which has brought to the investigation the sharpest x-ray vision ever used.

The first mystery is relatively close at hand, meaning on the order of 'only' tens of thousands of light-years away, within our own Milky Way. Astronomers see that the thin disk of our galaxy shines brightly in high energy, or 'hard', x-rays. Until now, no x-ray telescope could tell if this emission came from many individual stellar sources, such as neutron stars, or if it were spread through space.

An international team led by Dr. Ken Ebisawa of NASA's Goddard Space Flight Center pointed Chandra for a long look at a region of the galactic plane in the Scutum constellation. What they see in the highest resolution x-ray image ever taken is that 90% of the total emission is truly diffuse and coming from something spread through the plane of the galaxy. That something is most likely to be hot ionized gas, or plasma, where 'hot' is putting it mildly; to radiate in the x-ray band, the plasma would likely have to be at tens of millions of degrees.

But this only leads to another set of questions: Where did the plasma come from, and why does it stay confined to the narrow disk? Gas that hot has more than enough energy to escape the gravitational pull of the galaxy, so something must be confining it. Not knowing where it comes from or what's keeping it there means that astronomers are missing out on one of the most energetic processes going on in our own neighborhood. But they are never short on theories. That the gas is ionized may be a big hint. Dr. Ebisawa points out that ionized gas, where outer electrons have been stripped off the atoms leaving a net positive charge, could then be confined by a magnetic field.

Furthermore, though Chandra shows the emission as diffuse, it is by no means uniform. It is clumpy, and some patches of higher intensity are suspiciously close to suspected supernova remnants. These powerful explosions could well be the source of the plasma, but none of the many models of supernova evolution has yet distinguished itself from the pack by providing a testable explanation. The new data have settled only an observational uncertainty, leaving the real physics behind it to be discovered.

The unaccounted for 10% in their data appears to be far behind the gas in the galactic plane, and it is part of the second mystery, the component which is uniform in every direction. Unlike the cosmic microwave background (in brief, the diffuse heat left over from the Big Bang), this x-ray background is not explained by any theory of the origin of the universe. Nor can it be explained by assuming a uniform distribution of objects similar to what we have already resolved nearby; there aren't enough sources to explain the amount of radiation we see in the distance.

So two independent teams of astronomers recently pointed Chandra toward a couple of apparently empty pieces of sky for deep exposures of 500,000 and one million seconds respectively. Now known as the Chandra Deep Fields South and North, each is effectively a window from our own galaxy looking out to the more distant universe.

In this case, by contrast, the emission turns out to be due to many individual sources of x-rays, which have been identified as distant quasars by their signature energy spectra. Quasars are the very bright cores of some galaxies which spew out enormous amounts of high energy radiation, so much radiation that it can only be due to a black hole many millions of times the mass of the Sun. The average distance of these sources implies that they depict the universe as it was when much younger, only a few billion years old out of its current estimated age of 15 billion years.

This result has surprising implications for astronomers' understanding of the evolution of galaxies. What it says is that there were many more quasars in the past than are seen now, raising the big question: What happened to them? The implication is that quasars may be a stage of galaxy evolution, one through which even apparently ordinary galaxies like ours may have passed.

These galaxies would still harbor at their centres the supermassive black holes, only now the black holes are quiescent, perhaps because after a few billion years of gobbling up everything within reach, there is no more material near enough to feed them. The Chandra data strengthens considerably this evolutionary view that most normal galaxies may be essentially 'fossil quasars'.

But again, the solution to the observational mystery has raised yet more questions for astronomers. Foremost among them is the question of which came first, the galaxy or the black hole? Was the black hole born in the dense centre of an existing galaxy, or did the galaxy form around a 'seed' black hole? If the latter, where did the seed black holes come from? Though an observational uncertainty has been put to rest about the x-ray background, astronomers still have a lot to learn from the 'noise'.

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Author: Tess Jaffe

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