Milky Way black hole wakes up, eats dust cloud

Subodh Varma, TNN Oct 24, 2012, 01.17PM IST

(A black hole, three-to devour…)

NEW DELHI: The "gentle giant" black hole at the center of our galaxy, the Milky Way, has woken up and is munching on a dust cloud, scientists reported yesterday. This is the first time that direct observed evidence on the black hole called Sagittarius A* (pronounced Sagittarius A-star and abbreviated Sgr A*), has been collected.

Nasa's newest X-ray telescope, the Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June this year caught x-rays being emitted from the center of the Milky Way. Sgr A* is usually quiet and hence invisible because like all black holes it's very high density doesn't allow any light to escape from it. This time however it has been caught in the middle of a flare-up.

"We got lucky to have captured an outburst from the black hole during our observing campaign," said Fiona Harrison, the mission's principal investigator at the California Institute of Technology (Caltech) in Pasadena.

"These data will help us better understand the gentle giant at the heart of our galaxy and why it sometimes flares up for a few hours and then returns to slumber."

NuSTAR captures the highest-energy X-rays to produce focused images. For two days in July, it teamed up with NASA's Chandra X-ray Observatory, which sees lower-energy X-ray light; and the W.M. Keck Observatory atop Mauna Kea in Hawaii, which took infrared images to observe Sgr A*. The results were dramatic and unexpected.

Compared to giant black holes at the centers of other galaxies, Sgr A* is relatively quiet. Active black holes tend to gobble up stars and other fuel around them. Sgr A* is thought only to nibble or not eat at all, a process that is not fully understood, NASA scientists said. When black holes consume fuel -- whether a star, a gas cloud or, as recent Chandra observations have suggested, even an asteroid -- they erupt with extra energy.

In May this year scientists from Harvard and Johns Hopkins University reported their discovery of a feeding supermassive black hole in a galaxy some 2.5 billion light years away. It was gobbling up a red giant star consisting mainly of Helium.

In the case of Sgr A*, NuSTAR picked up X-rays emitted by consumed matter being heated up to about 100 million degrees Celsius and originating from regions where particles are boosted very close to the speed of light. Astronomers say these NuSTAR data, when combined with the simultaneous observations taken at other wavelengths, will help them better understand the physics of how black holes snack and grow in size.

""Astronomers have long speculated that the black hole's snacking should produce copious hard X-rays, but NuSTAR is the first telescope with sufficient sensitivity to actually detect them,"" said NuSTAR team member Chuck Hailey of Columbia University in New York City.

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