Decrease in the Brightness of the Cosmic X-ray and Soft Gamma-ray
  Background toward Clusters of Galaxies
  We show that Compton scattering by electrons of the hot intergalactic gas in
galaxy clusters should lead to peculiar distortions of the cosmic background
X-ray and soft gamma-ray radiation - an increase in its brightness at E<60-100
keV and a drop at higher energies. The distortions allow the most important
cluster parameters to be measured. The spectral shape of the distortions and
its dependence on the gas temperature, optical depth, and surface density
distribution law have been studied using Monte Carlo computations and confirmed
by analytical estimations. In the cluster frame the maximum of the background
decrease due to the recoil effect occurs at ~500-600 keV. The photoionization
of H- and He-like iron and nickel ions leads to additional distortions in the
background spectrum - a strong absorption line with the threshold at ~9 keV
(and also to an absorption jump at ~2 keV for cold clusters). The absorption of
intrinsic thermal radiation from the cluster gas by these ions also leads to
such lines. In nearby (z<1) clusters the line at ~2 keV is noticeably enhanced
by absorption in the colder (~10^6 K) plasma of their peripheral (~3 Mpc)
regions; moreover, the absorption line at ~1.3 keV splits off from it. The
redshift of distant clusters shifts the absorption lines in the background
spectrum (at ~2, ~9, and ~500 keV) to lower energies. Thus, in contrast to the
microwave background scattering effect, this effect depends on the cluster
redshift z, but in a very peculiar way. When observing clusters at z>1, the
effect allows one to determine how the X-ray background evolved and how it was
"gathered" with z. To detect the effect, the accuracy of measurements should
reach ~0.1%. We consider the most promising clusters for observing the effect
and discuss the techniques whereby the influence of the thermal gas radiation
hindering the detection of background distortions should be minimal.
