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Black
Holes in a radar trap:
Using the X-ray Satellite XMM-Newton researchers measure velocities
near the speed of light in the vicinity of cosmic mass monsters.
European astronomers succeeded for the first time to confirm the
signatures predicted near black holes by Albert Einstein's theory
of Relativity in the light of the cosmic X-ray background. The group
of scientists led by Günther Hasinger, director at the Max-Planck-Institute
for extraterrestrial Physics in Garching near Munich, could identify
the spectral fingerprint of iron atoms. They observed a strong,
relativistically smeared iron line in the average spectrum of roughly
100 active galaxies, whose X-ray light had been emitted when the
Universe was less than half of its current age.
The whole sky is filled with a diffuse, high energy glow: the cosmic
X-ray background. In the last years the astronomers could show,
that this radiation can almost completely be associated with individual
objects. Similarly, Galileo Galilei in the beginning of the 17th
century resolved the light of the Milky Way into individual stars.
The X-ray background originates in hundreds of millions of supermassive
black holes, which feed from matter in the centres of distant galaxy
systems. Because the black holes are accreting mass, we observe
them in the X-ray background during their growth phase. In today's
Universe, massive black holes are found in the centres of practically
all nearby galaxies.
When matter rushes down the abyss of a black hole, it speeds around
the cosmic maelstrom at almost the velocity of light and is heated
up so strongly, that it emits its "last cry for help"
in the form of high energy radiation, before it vanishes forever.
Therefore, the putatively invisible black holes are among the most
luminous objects in the universe, if they are fed well in the centres
of so called active galaxies. The chemical elements in the matter
emit X-rays of a characteristic wavelength and can therefore be
identified through their spectral fingerprint. Atoms of the element
iron are a particularly useful diagnostic tool, because this metal
is most abundant in the cosmos and radiates most intensely at high
temperatures.
In a way similar to the radar traps with which the police identify
speeding cars, the relativistic speeds of iron atoms circling the
black hole can be measured through a shift in the wavelength of
their light. Through a combination of the effects predicted by Einstein's
special and general theory of relativity, however, a characteristically
broadened, asymmetric line profile, i.e. a smeared fingerprint,
is expected in the X-ray light of black holes. Special relativity
postulates that moving clocks run slow, and general relativity predicts
that clocks run slow in the vicinity of large masses. Both effects
lead to a shift of the light emitted by iron atoms into the longer
wavelength part of the electromagnetic spectrum. However, if we
observe the matter circling in the accretion disk (see Figure 1)
from the side, the light from atoms racing towards us appears shifted
to shorter wavelengths and much brighter than that moving away from
us. These relativistic effects are stronger, the closer the matter
gets to the black hole. Because of the curved spacetime they are
strongest in fast rotating black holes. In the past years, measurements
of relativistic iron lines have been possible in a few nearby galaxies
– for the first time in 1995 with the Japanese ASCA satellite.
Now the researchers around Günther Hasinger of the Max-Planck-Institute
for extraterrestrial Physics, jointly with the group of Xavier Barcons
at the Spanish Instituto de Física de Cantabria in Santander
and Andy Fabian at the Institute of Astronomy in Cambridge, UK have
uncovered the relativistically smeared fingerprint of iron atoms
in the average X-ray light of about 100 distant black holes of the
X-ray background (see Figure 2). The astrophysicists utilized the
X-ray observatory XMM-Newton of the European Space Agency ESA. They
pointed the instrument to a field in the Ursa Major constellation
for more than 500 hours and discovered several hundred weak X-ray
sources. Because of the expansion of the Universe the galaxies move
away from us with a speed increasing with their distance and thus
their spectral lines all appear at different wavelength; the astronomers
had first to correct the X-ray light of all objects into the rest
frame of the Milky Way. The necessary distance measurements for
more than 100 objects were obtained with the American Keck-Telescope.
After having co-added the light from all objects, the researchers
were very surprised about the unexpectedly large signal and the
characteristically broadened shape of the iron line.
From the strength of the signal they deduced the fraction of iron
atoms in the accreted matter. Surprisingly, the chemical abundance
of iron in the "nutrition" of these relatively young black
holes is about three times higher than in our Solar system, which
had been created significantly later. The centres of galaxies in
the early Universe therefore must have had a particularly efficient
method to produce iron, possibly because violent star forming activity
"breeds" the chemical elements rather quickly in active
galaxies. The width of the line indicated that the iron atoms must
radiate rather close to the black hole, consistent with rapidly
spinning black holes. This conclusion is also found indirectly by
other groups, who compared the energy in the X-ray background with
the total mass of "dormant" black holes in nearby galaxies.
Original Publication
Alina Streblyanska, Günther Hasinger, Alexis Finoguenov, Xavier
Barcons, Silvia Mateos, Andy Fabian: XMM-Newton observations of
the Lockman Hole: III. A relativistic Fe line in the mean X-ray
spectra of type-1 and type-2 AGN, A&A Press Release AA/2004/1977,
astro-ph/0411340.
Contact
Prof. Dr. Günther Hasinger
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel.: +49 (89) 30000-3401
Fax: +49 (89) 30000-3403
E-Mail: grh@xray.mpe.mpg.de
Prof. Dr. Xavier Barcons
Instituto de Física de Cantabria (CSIC-UC)
Santander, Spain
Tel: +34 942 201461
E-Mail: barcons@ifca.unican.es
Prof. Dr. Andy Fabian
Institute of Astronomy
Cambridge, UK
Tel: +44 (1) 223-337548
E-Mail: acf@ast.cam.ac.uk
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