What does a black hole do for lunch?

If you guessed that a black hole swallows stars for lunch, you would be partly correct. The chances of a whole star falling into a black hole are really quite small, however. Just last week, a team of X-ray scientists found the best evidence of what is more likely to happen: the black hole destroys the star but only gulps down a small part of it. Using data from three different X-ray telescopes ROSAT, Chandra, and XMM Newton this team pieced together the tragic end of a star not too different from our own Sun.

In 1992, the German ROSAT telescope found a very bright X-ray source possibly associated with a distant galaxy. More recent observations with Chandra and XMM Newton showed that the source had faded dramatically, and what was left was at the center of the galaxy and had a signature (from the shape of the X-ray spectrum) of a massive black hole, one with a mass 100,000,000 times that of our Sun..

The scenario of what happened is that an ordinary star in this galaxy wandered too close to the black hole. Perhaps the stars orbit had been disturbed by yet another star in the central part of this galaxy. As the star sailed past the black hole, the enormous gravity stretched the star to the point of tearing it to pieces. In this process, called tidal disruption, most of the remains of the star can often escape the black hole. Some small fraction of the stars material is trapped, however, swirling around the black hole and being heated to enormous temperatures, producing the bright X-rays seen a decade ago. As the remnants of the star finally fell into the black hole, there was little left to produce X-rays, so the brightness dropped. All the observations seem consistent with this story.

You can see a nice animation of what appears to have happened at http://chandra.harvard.edu/photo/2004/rxj1242/animations.html

This week's question is provided by Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Laboratory for High Energy Astrophysics. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2006. His particular scientific interest is gamma-ray pulsars.