Chandra eyes low-temperature black hole.

From Atlanta, at a meeting of the American Astronomical Society

Black holes rank among the most intriguing objects in the universe. Now, astronomers have found a really cool one.

Homing in on the supermassive black hole at the heart of the Andromeda galaxy, the sharp eye of the Chandra X-ray Observatory has found that gas falling into the dense object has a temperature of a few million kelvins. That's the lowest temperature ever found for emissions from a galactic black hole. Sucking in hot, X-ray-emitting gas at nearly the speed of light, the black holes that scientists have previously studied have temperatures typically of about a billion kelvins.

Andromeda lies just 2 million light-years away from the Milky Way. The two galaxies are similar in size and shape, and astronomers have gathered evidence that each harbors a central black hole. Besides having a lower temperature, Andromeda's black hole emits radiation with a much lower ratio of X rays to radio waves than the Milky Way's black hole does.

Standard models can't explain this surprising behavior, reports Stephen S. Murray of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

One explanation for the cool observations may be that much less hot gas than cold gas is falling onto the black hole, says theorist Eliot Quataert of the Institute for Advanced Study in Princeton, N.J. That could occur if large pockets of hot gas boil and swirl around the black hole instead of falling directly onto it, he suggests.

Murray and his colleagues directed Chandra to image the nucleus of Andromeda several times last fall, and they found more than 100 distinct X-ray sources.

One source coincided with the position of the bright, pointlike object that astronomers believe to be the central black hole. Other X-ray telescopes have not been sharp enough to distinguish the source associated with Andromeda's black hole from other objects in the nucleus, Murray notes.

Because Chandra began observing the cosmos only 6 months ago, it's not yet clear whether the low-temperature black hole it spied in Andromeda is an oddball, Murray notes. Scrutinizing the cores of other nearby galaxies with Chandra, which Murray's team plans to do next year, should determine whether cool is commonplace, he says.

In the meantime, Chandra has also provided the first detailed X-ray look at the object believed to be a black hole at the Milky Way's core. The images show that the reputed monster is a bit of a wimp. Although it's hot, it's only one-fifth as bright as astronomers had expected, says Frederick K. Baganoff of the Massachusetts Institute of Technology.

The low luminosity may indicate that this black hole is swallowing its surroundings at a sluggish rate, says theorist Ramesh Narayan of Harvard-Smithsonian. Under such conditions, protons and ions trapped by the black hole reach enormous temperatures but can't radiate much of their heat (SN: 11/29/97, p. 346).