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Black hole sheds light on a galaxy
Light echo of a high-energy flash from a black hole first observed
A light echo occurs when interstellar gas is heated by radiation and
reacts by emission of light. An international team led by Stefanie
Komossa from the Max Planck Institute for extraterrestrial Physics
in Garching, Germany, has observed the light echo of an enormous X-ray
flare, which was almost certainly produced when a single star was
disrupted by a supermassive black hole. For the first time, the light
echo of such a rare and highly dramatic event could be observed in
great detail. The light echo not only revealed the stellar disruption
process, but it also provides a powerful new method for mapping
galactic nuclei (Astrophysical Journal Letters, Mai 2008).
The artistic view shows the light echo of a high-energy flash from
a black hole
When a star is disrupted by a black hole in the nucleus of a galaxy,
its debris is inevitably attracted and absorbed by the black hole.
This sudden increase in the accretion rate causes an abrupt burst
of ultraviolet and X-ray light because the gas from the disrupted
star becomes very hot. As the high-energy radiation travels through
the core of the galaxy it illuminates surrounding matter and so makes
it possible to probe regions of the galaxy that would otherwise be
"To study the core of a normal galaxy is like looking at the New York
skyline at night during a power failure: You can't learn much about
the buildings, roads and parks", says Stefanie Komossa. "The situation
changes, for example, during a fireworks display. It's exactly the same
when a sudden burst of high-energy radiation illuminates a galaxy."
However the astronomers had to hurry up and look through the telescope
at the right moment, because X-ray bursts don't last very long.
From the strength, the degree of ionization and the deduced velocities
of the rapidly varying emission lines, the physicists can tell in which
part of the galaxy they are emitted. The emission lines represent the
"fingerprints" of the atoms in the hot gases heated by the flare. The
galaxy with catalog name SDSSJ0952+2143 which was detected in December
2007 by Komossa and her team in the Sloan Digital Sky Survey archive
caught their attention because of its superstrong iron lines: the
strongest (relative to oxygen emission) that were ever observed in a
galaxy. In them the authors see an evidence for a molecular torus which
plays an important part in so-called unified models of active galaxies.
The unified model postulates that all active galaxies are made of identical
components and that the perceived differences are just due to the
different directions from which we view the galaxies. An important element
of this model is the molecular torus, which surrounds the black hole and
its accretion disk and covers them when viewed from certain directions.
Also the breadth of the spectral lines which the scientists measure is
influenced by the viewing direction and that means by the molecular torus.
Should the expectations of Komossa and her colleagues be confirmed, this
will be the first time that scientists have seen such a strong time-variable
signal from a molecular torus. From the light echo, the torus can be
mapped and its geometry inferred, something which has not been possible
up to now.
Along the same lines is the detection of variable emission in the infrared:
It can be interpreted as the "last cry for help" of the heated dusty torus
matter before the dust is destroyed by the flash.
In addition to the remarkably strong iron lines, the scientists also
noticed a very peculiar shape of the hydrogen emission lines which had
never been seen before. This line hints at activities of the disk of matter
around the black hole, which consists mainly of hydrogen. "Probably we
are seeing the debris of the disrupted star here which is just being accreted
by the black hole", explains Hongyan Zhou from the MPE, co-author of the
The recently-discovered light echo still continues and is being traced with
powerful telescopes. The burst itself has faded away. The first observations
with the X-ray satellite Chandra show measurable but already faint X-ray
light from the galactic nucleus.
"Reverberation-mapping of light echoes opens up new possibilities to study
galaxies", concludes Komossa. The team now wants to use this method to
explore the physical conditions in the circumnuclear material in active
and non-active galaxies.
ApJ Letters, 678, L13, 2008
Preprint: arXiv:0804.2670v1 [astro-ph]
Dr. Mona Clerico
Max Planck Institute for Astrophysics
and Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3980
Dr. Stefanie Komossa
Max Planck Institute for extraterrestrial Physics
Phone +49 89 30000-3577
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