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GIANT BLACK HOLE RIPS APART A STAR IN COSMIC
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
An enormously powerful X-ray outburst was detected from the
center of the galaxy RXJ1242-11, a billion lightyears
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
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
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
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.
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
on the state of matter in the environment of the
black hole, which otherwise
lingers in darkness.
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).
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).
Max-Planck-Institut für extraterrestrische Physik
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