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Fig. 1:
17-60 keV all-sky image obtained by INTEGRAL during four years of
operations. The sources in this image were artificially blurred and
the color map was stretched to make both strong and weak sources
readily visible. The concentration of sources along the mid-plane of
the image is due to neutron stars and stellar mass black holes in our
Galaxy, while the majority of sources located far away from the
Galactic plane are supermassive black holes in other galaxies. The
Cosmic X-ray Background is composed of the emission of tens of
millions of similar objects much further away from us. Superposed is
an Earth image by ESA/EUMETSAT's Meteosat satellite. Blocking the
emission from a distant source by the Earth disk allows astronomers to
measure the intensity of the X-ray and gamma-ray background. An
angular size of the Earth as seen from Integral during actual
observations was smaller than shown in the image.
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Fig. 2:
Comparison of the CXB spectrum measured by INTEGRAL (blue points with
error bars) with a model in which AGNs experience luminosity
downsizing since z~1.5 till z=0, as suggested by deep extragalactic
X-ray surveys, while the fractions of obscured and unobscured
AGNs remain the same as measured locally by INTEGRAL. The two upper
curves and the shaded regions around them
reflect the current uncertainty in the AGN evolution at high redshifts
(z>1-1.5). Also shown are the contributions to the CXB from AGNs with
different degrees of obscuration.
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Supermassive black holes millions and billions times heavier than our
Sun are lurking in the centers of all galaxies. Many of these black
holes are dormant now, but each had a period of rapid growth in its
history when a vast amount of radiation was emitted. Such objects are
observed as Seyfert galaxies in our vicinity and as powerful quasars in
the distant Universe. All together the growing black holes are
believed to be making a dominant contribution to the "cosmic X-ray
background" (CXB) - hard radiation filling the space around us.
Understanding the nature of the CXB means that we know its flux and that we
can account for all of it in terms of known populations of objects. At
low energies (below few keV) deep surveys with large X-ray telescopes
like XMM-Newton and Chandra already resolve 90% of the CXB, directly
counting of order of 1000 sources per square degree of the sky. At
energies higher than 10 or 20 keV such an exercise is far beyond the
capabilities of existing instruments and other techniques have been
employed by the INTEGRAL observatory to study the CXB above 20 keV.
First of all, along with an extensive study of the Galactic plane
INTEGRAL used a fraction of its observing time to map the whole sky in
the 17-60 keV band, identifying more than a hundred of active black
holes (AGNs) in the nuclei of nearby galaxies. While these objects
contribute at most few per cent of the CXB, the all sky survey by
INTEGRAL provides un unbiased sample of local active black holes,
including those surrounded by large masses of cold gas which absorbs soft
X-ray
radiation, making such objects invisible below few keV.
Using this survey, the researchers have studied the distribution of
nearby AGNs in hard X-ray luminosity and
demonstrated that the fraction of obscured objects is high (~70%) in
the faint end of the AGN luminosity function and low (~25%) in its
bright end.
Secondly, aiming at measuring the total CXB flux, the INTEGRAL
observatory spent several days looking directly at the Earth disk. At
first glance it looks strange that in order to study emission from
extragalactic sources one has to point the telescope at the Earth
instead. In fact the Earth works as a giant shield, which blocks the
emission of a distant source and causes a decrease in the flux seen by
INTEGRAL. By measuring this decrease the combined emission from all
distant sources, no matter how weak they are, can be measured. This is
schematically shown in Fig. 1, where the Earth disk is superposed onto
the all sky 20-50 keV map obtained by INTEGRAL. This task is not
trivial since the Earth itself is a source of hard X-ray radiation,
produced by cosmic rays interacting with the Earth atmosphere.
Now, knowing the total flux of the CXB and the detailed properties of
the local population of active black holes, scientists are able to
learn more on how such black holes have been evolving from early
times (corresponding to redshifts z~1.5) till today (z=0, Fig. 2). It
turns out that both the amplitude and spectral distribution of the CXB
can be explained if the AGN population as a whole has
experienced pure luminosity downsizing (from bright quasars to
relatively weak Seyfert galaxies), as suggested by deep extragalactic X-ray
surveys, while its other key properties such as the ratio of obscured
to unobscured AGNs have not changed. This has important implications
for theoretical models trying to unify different types of AGNs based
on geometrical orientation and physical properties.
E.Churazov, R.Sunyaev, S.Sazonov, M.Revnivtsev, R.Krivonos
Publications
Churazov et al.:
"Integral observations of the cosmic X-ray background in
the 5-100 keV range via occultation by the Earth",
Astron. & Astrophys.;
 astro-ph/0608250
Sazonov et al.:
"Hard X-ray emission of the Earth's atmosphere: Monte Carlo
simulations",
Astron. & Astrophys.;
astro-ph/0608253
Churazov et al.:
"Earth X-ray albedo for CXB radiation in the 1-1000 keV
band",
Astron. & Astrophys., in press;
astro-ph/0608252
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