Samplings: July–August 2008

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©iStockphoto.com/Michael Puerzer

The space surrounding a black hole at a galaxy’s center normally radiates lots of X-rays, yet the vicinity of the black hole called SagittariusA that lies at the hub of our own Milky Way is unusually dim. It wasn’t always so: 300 years ago, astronomers now say, Sagittarius A flared up with X-rays, blazing out a million times more radiation than it does today. How were the astronomers able to peer back in time? Apparently, through a clever trick with mirrors. Next to the black hole, at a distance of 300 light-years, is a cloud of gas and dust that reflects X-rays. Because radiation takes 300 years longer to travel from black hole to cloud to Earth than directly from black hole to Earth, the cloud now shows black-hole radiation as it would have appeared 300 years ago, say Tatsuya Inui and three colleagues, all from Kyoto University in Japan. Another possibility is that the cloud emits its own X-rays as charged particles collide with it—but then the emissions would remain constant from year to year. By combining years of observations from several X-ray telescopes, Inui’s team discovered a steep drop in radiation coming from the cloud over a ten-year period. Such instability is a sign of Sagittarius A’s waning X-ray flare. Magic tricks aside, the black hole still holds mystery. Nobody knows why it’s quiescent now—or whether it will spring to life again someday. (Publications of the Astronomical Society of Japan)