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GIANT BLACK HOLE RIPS APART A STAR IN COSMIC REALITY SHOW

prepared by S. Komossa


Astronomers now have the first strong evidence of a giant supermassive black hole ripping apart a star at the center of a distant galaxy, a process long predicted by theory (e.g. Rees, Nature 333, 523 (1988)).

The new finding is based on observations made with some of the major astronomical observatories in Earth orbit: the X-ray satellites Chandra and XMM-Newton and the Hubble Space telescope, combined with earlier data from the X-ray mission ROSAT.

An enormously powerful X-ray outburst was detected from the center of the galaxy RXJ1242-11, a billion lightyears from Earth. The center of the galaxy suddenly flared up in X-rays. At maximum, it was thousands of times brighter than all its billions of stars taken together. Yet, in visible light (as confirmed with Hubble), this is just a normal, inconspicuous galaxy (which, apart from the giant X-ray flare, does not show any kinds of activity).
When re-observed with Chandra and XMM, the X-ray emission had dropped dramatically, but not yet faded away completely. We still see some "afterglow" of the once bright flare, coming straight from the center of the galaxy. The characteristics of the X-ray radiation - the X-ray "spectrum" - was measured with XMM-Newton. ESA's satellite was pointed at this galaxy immediately after the flare afterglow was detected with NASA's Chandra, in order to catch the X-ray signal before it had faded away completely. The X-ray radiation shows a wide spread in energy - the characteristic emission of matter close to a black hole.

Like putting together the pieces of a puzzle in a detective story, these new observations now tell us what happened at the center of this galaxy:
A doomed star ventured to close to a massive black hole. Once close enough, the star starts to feel extreme tidal forces, exerted by the black hole. These first stretch the star, then finally completely rip it apart. Part of the stellar debris is pulled toward the black hole. In the extreme conditions close to the "monster" it heats itself up enormously, to millions of degrees. Before disappearing into the black hole forever, it emits a brilliant flare of X-ray radiation - a "last cry for help" of the dying star.

In a much more gentle form, we also experience tidal forces on Earth, when moon and sun "push around" the waters of the oceans, causing the tides. Tidal forces also destroyed comet Shoemaker-Levy, when it crashed into Jupiter.

Only some fraction of the star was actually "digested" by the black hole. Yet, the energy set free when the stellar debris plunges into the black hole, is tremendous: about 1044 Watt s.

How would it look like if such an event occurred at our own Galactic center, as would be expected to happen once every 10000 years ?
The center of our Milky Way would flare up to become about 100 billion times brighter than it is now! Such an event would pose no threat to life on Earth. However, the detectors aboard X-ray observatories like Chandra and XMM-Newton would be destroyed, if they directly looked into such a bright source.

Tidal disruptions of stars are an unavoidable consequence, if black holes reside at the centers of non-active galaxies. Detections of such events therefore provide a totally independent route to search for black holes. The new results also help us to understand how black holes are "fed"; i.e. how they grow to their sometimes enormous masses. The black hole at the center of the galaxy RXJ1242-11 has a mass of about a hundred million times the mass of our sun! Stellar tidal disruption (in addition to accretion of interstellar matter and black hole - black hole merging) is one of the three major processes thought to fuel black holes. We now have a clearer picture on how black holes evolve and how they can affect surrounding stars - which can be quite dramatic, as we have seen.

In the future, giant X-ray flares like the one now observed will enable us to detect black holes at the centers of normal galaxies out to very large distances. Planned future X-ray surveys which will monitor large fractions of the sky, will easily pick up these events. Then, we will be alert and alarm astronomers all around the globe to monitor this fascinating experiment which nature performs for us.
In particular, the bright flare will illuminate its environment and will cause a giant "light echo" when traveling through the core of the galaxy. The same way somebody lost in darkness can get orientation with a flash-light, the light-echo of the bright flare provides astronomers with valuable information on the state of matter in the environment of the black hole, which otherwise lingers in darkness.


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Fig. 1: A star is ripped apart by the tidal forces of a massive black hole (right panel). Part of the stellar debris is then accreted by the black hole (middle panel). This causes a luminous flare of radiation which fades away as more and more of the matter disappears into the black hole (left panel).


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Fig. 2: X-ray "afterglow" (blue) of the once bright flare, superposed on an optical image of the galaxy (orange).




Authors of publication: S. Komossa, J. Halpern, N. Schartel, G. Hasinger, M. Santos-Lleo, P. Predehl; in press with ApJ Letters (March 1 issue).

Contact:

Stefanie Komossa
Max-Planck-Institut für extraterrestrische Physik
phone: 49 89 30000 3577
email: skomossa@mpe.mpg.de

Günther Hasinger
Max-Planck-Institut für extraterrestrische Physik
phone: 49 89 30000 3401
email: ghasinger@mpe.mpg.de