Black Hole Rips Star Apart

Tidal disruption of star
An artist's illustration of a supermassive black hole tidally disrupting a star. 1. The star falls from a distance toward the hole. 2. As the star draws near, it becomes stretched into an ellipsoid by the differing strength of the hole's gravity on its near and far sides. 3. If the star swings too close, the differences between the hole's pull on the star's different parts grow greater than the star's self-gravity, tearing it apart into an X-ray-hot gas.
Courtesy NASA / CXC / M. Weiss.
Yes, black holes can be just as mean and nasty as they're made out to be in cheesy science fiction. A supermassive black hole in a galaxy 700 million light-years away has torn to shreds an unfortunate star that ventured too close to it, astronomers declared today. The violence of the event created a decade-long X-ray flare that first caught astronomers' attention in 1992. Last year they were able to study its fading remains using the XMM-Newton and Chandra X-ray satellites, and this afternoon, at a NASA press conference, they described what they think must have happened.

Astronomers have been hoping to catch such an event for a long time. The central black hole in a typical, Milky-Way-like galaxy ought to swallow a random star only about once every 10,000 years. But the German ROSAT X-ray satellite surveyed so many galaxies during its lifetime that researchers realized that a few star-swallowings might well have been recorded. Last year a group identified two likely cases of this happening (Sky & Telescope: August 2003, page 24). Now a team led by Stefanie Komossa (Max Planck Institute for Extraterrestrial Physics, Germany) says it has fully confirmed one of these events.

The flareup occurred in the faint galaxy RX J1242–11 in Virgo. The nucleus of this galaxy normally shows no trace of activity. Nevertheless, the X-ray outburst was one of the strongest ever seen from any galactic nucleus — brightening it enough to equal, temporarily, the X-ray luminosity of a full-fledged quasar. The flare's power, its sudden onset (two years earlier nothing was happening), and other characteristics match what would be expected for a solar-mass star swinging fatally close to a 100-million-solar-mass black hole. Said team member Guenther Hasinger (Max Planck Institute), "We are convinced that this is the only explanation for these events."

The Chandra X-ray Observatory provided the high resolution needed to place the flare at the center of the galaxy, where a supermassive black hole is expected to reside. The European Space Agency's XMM-Newton obtained high-quality X-ray spectra; these showed the temperature and other signatures expected of large amounts of matter close to a supermassive, galaxy-core black hole. Independent commentator Alexei Filippenko (University of California, Berkeley) said the group's work "provides the definitive evidence that stars are being ripped apart occasionally by black holes."

Komossa and her colleagues estimate that only between 1 and 25 percent of the star's mass ended up forming a hot accretion disk spiraling around the hole on its way to getting swallowed. The rest of the star's remains swung clean around the hole and were flung back out to deep space.

More information and animations are available in a Chandra press release. The researchers' paper will appear in the March 1st Astrophysical Journal Letters.

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