Jet set. Hot gas explodes out of a microquasar, heading toward the top and bottom of the image.

Vanishing X-rays Betray Black Hole

Many galaxies and quasars are thought to have a giant black hole at their center that sucks in matter and occasionally ejects high-speed streams of ultrahot gas. Now, a "microquasar" in our own galaxy, thought to be centered on a black hole just a few times more massive than our sun, is showing similar violent behavior, but much faster than the billion-year time scale of quasars. What's more, a paper scheduled to appear in the February issue of Astronomy and Astrophysics describes tantalizing evidence that this microquasar really is centered on a black hole.

Jets of matter coming from a microquasar called GRS 1915, just 40,000 light-years from Earth, were detected 3 years ago by astronomer Felix Mirabel of France's Atomic Energy Commission at Saclay and colleagues, who used the Very Large Array radiotelescope of the U.S. National Radio Astronomy Observatory in New Mexico. Last month, the jets were confirmed when a network of British radiotelescopes called Merlin tracked the flares coming from GRS 1915. "The jets appear to be generated in less than a day, and we see the material speeding out over something like 2 weeks," says Rob Fender of the University of Amsterdam in the Netherlands

Similar processes take place in quasars, where black holes are billions of times more massive, says Michael Garcia of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But with the microquasar "you can actually see things happen in a minute. You can't see things like that happen in a quasar: It takes billions of years."

Mirabel has also been keeping an eye on GRS 1915, in particular on x-ray flares thought to be caused by globules of matter being heated to enormous temperatures as they are sucked into the black hole. Mirabel and his team found that these x-ray flares often stopped abruptly within a few seconds. "We observed a sudden drop in the x-ray flux" that wouldn't be observed if the material landed on the surface of a star, he says. Mirabel argues that the only explanation for such an abrupt switching off of the x-rays is that the globules of matter must be suddenly disappearing beyond the "event horizon" of the black hole. This scenario is "a very good model," says Garcia, although he notes that there is no "absolute direct evidence."

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