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Over the recent years a wealth of observational data has provided
further strong support to the picture in which the CXB is a superposition of
AGN. In particular, the bulk of the CXB at energies below several
keV has been directly resolved by X-ray telescopes into individual
AGN. However, the peak of the CXB spectral energy distribution is
located near 30 keV where 99% of the background emission remains
unresolved and it was never directly demonstrated that the sum of the hard
X-ray (between ten and several hundred keV) spectra of all AGN
residing in a given volume of Universe is indeed compatible with the measured
spectral energy distribution of the CXB
(see
Research Highlight October 2006
on a recent CXB measurement by INTEGRAL).
Such crucial comparison has now been made by MPA researchers using
all-sky hard X-ray surveys performed by the RXTE
(Research Highlight February 2004)
and INTEGRAL
(Research Highlight October 2006
and
Research Highlight May 2007)
observatories.
The goal was to determine the cumulative spectrum of
the local (at redshift z<0.1) AGN population in the broad energy band
3-300 keV. To this end a stacking analysis was performed of the
spectra of 84 AGN detected by the PCA instrument aboard RXTE and of 68
AGN detected by the IBIS/ISGRI instrument aboard INTEGRAL, properly taking into
account the space densities of AGN with different luminosities
(using the well-known 1/Vmax technique).
The resulting summed spectrum (Figure 1) peaks at 50-80 keV and
rolls over both below 20 keV (due to photoabsorption in
obscured AGN which outnumber unobscured ones) and above 100-200
keV. This local cumulative AGN spectrum proves to be consistent,
albeit within the significant statistical and systematic
uncertainties, with the CXB spectral energy distribution in both shape
and normalization (Figure 2) if the strong "cosmic downsizing" of AGN
between redshifts z~1 and z=0, known from deep X-ray surveys with
Chandra and XMM-Newton, has occured without significant changes in the
average hard X-ray spectral shape of AGN.
The implication is that the popular concept of the CXB
being a superposition of AGN is grossly correct. Improved measurements
of the cumulative spectral distribution and cosmological evolution of
AGN by current and future X-ray and hard X-ray astronomy missions will
provide tighter constraints on the cosmic history of massive black
hole growth and AGN unification schemes.
S. Sazonov, M. Revnivtsev, R. Krivonos, E. Churazov, R. Sunyaev
Reference:
S. Sazonov, R. Krivonos, M. Revnivtsev, E. Churazov, and R. Sunyaev,
Cumulative hard X-ray spectrum of local AGN: a link to the cosmic
X-ray background, submitted to Astronomy and Astrophysics,
arXiv0708.3215
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