Abstract.We report the association of the recently discovered hard X-ray source IGR J17475−2822 with the giant molecular cloud Sgr B2 in the Galactic Center region. The broad band (3-200 keV) spectrum of the source constructed from data of different observatories strongly supports the idea that the X-ray emission of Sgr B2 is Compton scattered and reprocessed radiation emitted in the past by the Sgr A source. We conclude that 300-400 years ago Sgr A was a low luminosity (L ≈ 1.5 × 10 39 erg s −1 at 2-200 keV) AGN with a characteristic hard X-ray spectrum (photon index Γ ≈ 1.8). We estimate the mass and iron abundance of the Sgr B2 scattering gas at 2 × 10 6 M (r/10 pc) 2 and 1.9 solar, respectively (where r is the radius of the cloud).

Electron scattering induces a polarization in the cosmic microwave background (CMB) signal measured in the direction of a galaxy cluster owing to the presence of a quadrupole component in the CMB temperature distribution. Measuring the polarization towards distant clusters provides the unique opportunity to observe the evolution of the CMB quadrupole at moderate redshifts, z∼0.5–3. We demonstrate that for the local cluster population the polarization degree will depend on the cluster celestial position. There are two extended regions in the sky, which are opposite to each other, where the polarization is maximal, ∼0.1(τ/0.02) μK in the Rayleigh–Jeans part of the CMB spectrum (τ being the Thomson optical depth across the cluster). This value exceeds the polarization introduced by the cluster transverse peculiar motion if vt<1300 km s−1. One can hope to detect this small signal by measuring a large number of clusters, thereby effectively removing the systematic contribution from other polarization components produced in clusters. These polarization effects, which are of the order of (vtc)2τ, (vtc)τ2 and (kTemec2)τ2, as well as the polarization owing to the CMB quadrupole, were previously given by Sunyaev and Zel’dovich for the Rayleigh–Jeans part of the spectrum. We fully confirm their earlier results and present exact frequency dependences for all these effects. The polarization degree is considerably higher in the Wien region.

The photon frequency distribution that results from single Compton scattering of monochromatic radiation on thermal electrons is derived in the mildly relativistic limit. Algebraic expressions are given for (1) the photon redistribution function, K(ν, Ω → ν ′ , Ω ′ ), and (2) the spectrum produced in the case of isotropic incident radiation, P(ν → ν ′ ). The former is a good approximation for electron temperatures kT e ∼ < 25 keV and photon energies hν ∼ < 50 keV, and the latter is applicable when hν(hν/m e c 2 ) ∼ < kT e ∼ < 25 keV, hν ∼ < 50 keV. Both formulae can be used for describing the profiles of X-ray and low-frequency lines upon scattering in hot, optically thin plasmas, such as present in clusters of galaxies, in the coronae of accretion disks in X-ray binaries and active galactic neclei (AGNs), during supernova explosions, etc. Both formulae can also be employed as the kernels of the corresponding integral kinetic equations (direction-dependent and isotropic) in the general problem of Comptonization on thermal electrons. The K(ν, Ω → ν ′ , Ω ′ ) kernel, in particular, is applicable to the problem of induced Compton interaction of anisotropic low-frequency radiation of high brightness temperature with free electrons in the vicinity of powerful radiosources and masers. Fokker-Planck-type expansion (up to fourth order) of the integral kinetic equation with the P(ν → ν ′ ) kernel derived here leads to a generalization of the Kompaneets equation. We also present (1) a simpler kernel that is necessary and sufficient to derive the Kompaneets equation and (2) an expression for the angular function for Compton scattering in a hot plasma, which includes temperature and photon energy corrections to the Rayleigh angular function.

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