Date: 14 January 2000
Armed with the potent imaging power of the Chandra space telescope, a team of scientists claims to have answered a 37-year old question that has poked at astronomers since humanity’s first foray into high-energy astronomy: the mystery of the X-ray background radiation.
The Chandra telescope has given these scientists a glimpse at the source of the diffuse X-ray glow that seems to permeate the entire universe, said Richard Mushotzky, a researcher at NASA’s Goddard Space Flight Center.
That glow is produced by a "new class of objects" scattered throughout the universe that emit X-ray radiation, he said. The objects aren’t exactly unknown, but they were never before shown to be the sources of X-ray radiation, said Mushotzky, who leads a group of astronomers that used Chandra to study the puzzling universal X-ray glow.
He announced the discovery here Thursday at the meeting of the American Astronomical Society.X-ray astronomy must be done from space because Earth’s atmosphere blocks out X-ray radiation – the energy that comes from some of the hottest, most energetic objects in the universe.
The very first rocket flight that was capable of X-ray astronomy observed a diffuse glow that seemed to permeate the universe in every direction. The phenomenon was dubbed the X-ray background, and it has defied definitive explanation ever since.
Chandra’s ability to see objects 100 times fainter than previous X-ray instruments, though, means the telescope was, in some sense, built for the task, Mushotzky said.
The telescope allowed the astronomers to see the points that were emitting the peculiar brand of X-rays that seemed to come from everywhere and nowhere at all, and they discovered that these points were found throughout the universe -- perhaps 100 million of them spread over the entire sky, Mushotzky said.
"The heart of the background is made up of a rich assortment of X-ray sources which could not have been discovered by previous satellite missions," said Amy Barger, a University of Hawaii astronomer who worked to find optical counterparts to the X-ray sources.
Using ground-based telescopes, Barger chased down the source of many of the X-ray points and found that they could be classified into four categories.
Some of the X-rays come from galaxy clusters, and some from the energetic active galactic nuclei known as quasars. A third source seems to be bright galaxies, which look in most respects like any normal run-of-the-mill galaxy. The final category of sources simply can’t be seen at visible wavelengths, so Barger calls this class "optically-faint objects."
Most likely, the sources in all these cases are super-massive black holes, either in the centers of galaxies or galaxy clusters, or hidden from view by surrounding clouds of dust, Barger said.
She reasons that the optically faint sources might be black holes so shrouded by gunk that visible light is blocked out while X-rays escape to scatter throughout the universe.
Astronomers who worked on the problem are relieved, and are hailing the discovery as something big. Still, the answer is not a complete surprise, for many scientists have suggested that the background must be created by a diverse array of objects throughout the universe. Still, until now it has never been proved, Mushotzky said.
For years the uncertainty of the X-ray background's source has been an irritant to cosmologists, said Virginia Trimble, of the University of California at Irvine.
Trimble, who is not affiliated with the team that announced the discovery said that knowing the sources of the X-ray radiation should help cosmologists develop better understandings about the overall shape and structure of the universe. One of the great questions of the day, she said, is why is the universe lumpy, and not smooth and homogeneous throughout. The new discovery could help answer those questions, Trimble said.
"The X-ray background has subtle fluctuations in it. You could use those to trace large-scale structure if only you knew what was making that X-ray background. We now know what is making that X-ray background," she said.
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