The black holes that form from collapsed stars  are stories in contradiction. They’re small but heavy. They have a surface, but it’s not something you could touch. And they can be both completely dark and overpoweringly bright.
Astronomers have been studying an example of that last contradiction in M31, the Andromeda galaxy. They discovered an object that produced more X-ray energy than the rest of the galaxy combined. Yet that energy came from a tiny region of space — a region encircling a black hole.
The object is known as an Ultraluminous X-ray source. It was discovered in 2009 by an X-ray telescope in space. Since then, astronomers have watched with several space telescopes as the object slowly faded a bit.
The X-rays came from a disk of superhot gas around a black hole that’s about 13 times as massive as the Sun. The black hole is “stealing” gas from a fairly average companion star. So much gas was spiraling in that the black hole couldn’t swallow it all, so the gas piled up. This logjam caused the gas to get hotter and hotter, so it emitted huge amounts of X-rays.
Eventually, something broke the logjam. Perhaps the X-rays blew some of the infalling gas away from the black hole, or perhaps something else happened. Whatever the case, the X-rays faded a bit. But such outbursts tend to repeat, so the object in M31 may one day flare up again as the black hole gorges itself on gas from its companion.
More about M31 tomorrow.
Script by Damond Benningfield, Copyright 2012
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