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Black Holes Encyclopedia

Milky Way's Quiet Black Hole Flared in Past

(From the May/June 2005 issue of StarDate magazine)

Oliver Cromwell had Charles I beheaded, toppling the British monarchy. An outbreak of plague shut down Cambridge University, leaving 22-year-old student Isaac Newton to return home and spend his leisure hours creating, among other things, the theory of gravity. And had they existed then, space-borne gamma-ray telescopes would have seen the black hole at the center of our Milky Way galaxy flare to life.

Each of these events occurred about 350 years ago -- the last according to a team of scientists working with Europe's gamma-ray satellite Integral. The group, led by Mikhail Revnivtsev, says its findings indicate that the black hole released a million times more energy then than it does today. The flare-up happened when a large amount of material fell into the black hole and was crushed and heated to millions of degrees as it neared the event horizon, or point of no return.

Integral data show a flood of X-rays and gamma-rays from the black hole slamming into a giant cloud of gas, called Sagittarius B2, that sits 350 light-years from the galactic center. The X-rays and gamma-rays are absorbed by the gas in Sagittarius B2, leaving behind a signature that's easy to read, the astronomers said.

"We are now seeing an echo from a sort of natural mirror near the galactic center -- the giant cloud Sagittarius B2 simply reflects gamma-rays emitted by [the black hole] in the past," Revnivtsev said. -- Rebecca Johnson