S. Yu. Sazonov

Radiative feedback from quasars and the growth of massive black holes in stellar spheroids

We discuss the importance of feedback via photoionization and Compton heating on the co-evolution of massive black holes (MBHs) at the centre of spheroidal galaxies, and their stellar and gaseous components. We first assess the energetics of the radiative feedback from a typical quasar on the ambient interstellar medium (ISM). We then demonstrate that the observed M BH -σ relation could be established following the conversion of most of the gas of an elliptical progenitor into stars, specifically when the gas-to-stars mass ratio in the central regions has dropped to a low level ∼0.01 or less, so that gas cooling is no longer able to keep up with the radiative heating by the growing central massive black hole (MBH). A considerable amount of the remaining gas will be expelled and both MBH accretion and star formation will proceed at significantly reduced rates thereafter, in agreement with observations of present-day ellipticals. We find further support for this scenario by evolving over an equivalent Hubble time a simple, physically based toy model that additionally takes into account the mass and energy return for the spheroid evolving stellar population, a physical ingredient often neglected in similar approaches.

Quasars: the characteristic spectrum and the induced radiative heating

Using information on the cosmic X-ray background and the cumulative light of active galactic nuclei (AGN) at infrared wavelengths, the estimated local mass density of galactic massive black holes (MBHs) and published AGN composite spectra in the optical, UV and X-ray, we compute the characteristic angular-integrated, broad-band spectral energy distribution of the average quasar in the universe. We demonstrate that the radiation from such sources can photoionize and Compton heat the plasma surrounding them up to an equilibrium Compton temperature (T C ) of 2 x 10 7 K. It is shown that circumnuclear obscuration cannot significantly affect the net gas Compton heating and cooling rates, so that the above T C value is approximately characteristic of both obscured and unobscured quasars. This temperature is above typical gas temperatures in elliptical galaxies and just above the virial temperatures of giant ellipticals. The general results of this work can be used for accurate calculations of the feedback effect of MBHs on both their immediate environs and the more distant interstellar medium of their host galaxies.

Hard X-ray view of the past activity of Sgr A in a natural Compton mirror

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 Asource. We conclude that 300-400 years ago Sgr Awas 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

An apparently normal γ -ray burst with an unusually low luminosity

Much of the progress in understanding γ-ray bursts (GRBs) has come from studies of distant events (redshift z ≈ 1). In the brightest GRBs, the γ-rays are so highly collimated that the events can be seen across the Universe. It has long been suspected that the nearest and most common events have been missed because they are not as collimated or they are under-energetic (or both). Here we report soft γ-ray observations of GRB 031203, the nearest event to date (z = 0.106; ref. 2). It had a duration of 40 s and peak energy of >190 keV, and therefore appears to be a typical long-duration GRB. The isotropic γ-ray energy of ≤1050 erg, however, is about three orders of magnitude smaller than that of the cosmological population. This event—as well as the other nearby but somewhat controversial GRB 980425—is a clear outlier from the isotropic-energy/peak-energy relation and luminosity/spectral-lag relations that describe the majority of GRBs. Radio calorimetry shows that both of these events are under-energetic explosions. We conclude that there does indeed exist a large population of under-energetic events.

Statistical properties of local active galactic nuclei inferred from the RXTE 3-20 keV all-sky survey

We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keV band at Galactic latitude |b| > 10 ◦ during the RXTE slew survey (XSS, Revnivtsev et al. 2004), and utilize it to study the statistical properties of the local population of AGNs, including the X-ray luminosity function and absorption distribution. We find that among low X-ray luminosity (L3−20 10 41 erg s −1 estimated here is smaller than the earlier estimated total X-ray volume emissivity in the local Universe, suggesting that a comparable X-ray flux may be produced together by lower luminosity AGNs, non-active galaxies and clusters of galaxies. Finally, we present a sample of 35 AGN candidates, composed of unidentified XSS sources.

IGR J16318-4848: An X-ray source in a dense envelope?

We analyze in detail the ASCA observations of the hard X-ray source IGR J16318-4848, which was recently discovered by the INTEGRAL observatory (Courvoisier et al. 2003). The source has an anomalously hard spectrum in the energy range 0.5–10 keV and is virtually undetectable below 4 keV because of strong photoabsorption (nHL>4×1023 cm−2). The Kα line of neutral or weakly ionized iron with an equivalent width of ∼2.5 keV dominates in the energy range 4–10 keV. There is also evidence for the presence of a second line at energy ∼7 keV. Our analysis of archival observational data for the infrared counterpart of IGR J16318-4848 that was discovered by Foschini et al. (2003) revealed the source in the wavelength range 1–15 µm. Available data suggest that the object can be an X-ray binary system surrounded by a dense envelope. The source may be a high-mass X-ray binary similar to GX 301-2. We believe that IGR J16318-4848 can be the first representative of a hitherto unknown population of strongly absorbed Galactic X-ray sources that could not be detected by previous X-ray observatories.

Polarization of resonance X-ray lines from clusters of galaxies

It is known that resonant scattering can distort the surface brightness profiles of clusters of galaxies in X-ray lines. We demonstrate that the scattered line emission should be polarized and possibly detectable with future X-ray polarimeters. Spectrally resolved mapping of a galaxy cluster in polarized X-rays could provide valuable independent information on the physical conditions, in particular element abundances and the characteristic velocity of small-scale turbulent motions, in the intracluster gas. The expected degree of polarization is of the order of 10 per cent for the richest regular clusters (e.g. Coma) and clusters whose X-ray emission is dominated by a central cooling flow (such as Perseus and M87/Virgo).

Optical identifications of five INTEGRAL hard X-ray sources in the Galactic plane

The results of optical identifications of five hard X-ray sources in the Galactic plane from the INTEGRAL all-sky survey are presented. The X-ray data on one source (IGR J20216+4359) are published for the first time. The optical observations were performed with the 1.5-m RTT-150 telescope (Turkish National Observatory, Antalya, Turkey) and the 6-m BTA telescope (Special Astrophysical Observatory, Nizhny Arkhyz, Russia). A blazar, three Seyfert galaxies, and a high-mass X-ray binary are among the identified sources.

Resonant scattering in galaxy clusters for anisotropic gas motions on various spatial scales

The determination of the characteristic amplitudes and directions of hot gas motions in galaxy clusters from observations of the brightest resonance lines is discussed. Gas motions affect (i) the spectral line shape through the Doppler effect and (ii) the distortions of the radial surface brightness profiles in lines during resonant scattering. Radiative transfer calculations have been performed by the Monte Carlo method in the FeXXV resonance line at 6.7 keV for the Perseus cluster A426. As a result, (a) radial motions have been shown to reduce the scattering efficiency much more dramatically than purely tangential motions; (b) large-scale gas motions have been shown to affect weakly the scattering efficiency; and (c) the uncertainty in measuring the characteristics of gas motions using resonant scattering has been estimated from existing and future observations of clusters.

Polarization of X-ray lines from galaxy clusters and elliptical galaxies-a way to measure the tangential component of gas velocity

We study the impact of gas motions on the polarization of bright X-ray emission lines from the hot intercluster medium. The polarization naturally arises from resonant scattering of emission lines owing to a quadrupole component in the radiation field produced by a centrally peaked gas density distribution. If differential gas motions are present, then a photon emitted in one region of the cluster will be scattered in another region only if their relative velocities are small enough and the Doppler shift of the photon energy does not exceed the linewidth. This affects both the degree and the direction of polarization. The changes in the polarization signal are in particular sensitive to the gas motions perpendicular to the line of sight. We calculate the expected degree of polarization for several patterns of gas motions, including a slow inflow expected in a simple cooling flow model and a fast outflow in an expanding spherical shock wave. In both cases, the effect of non-zero gas velocities is found to be minor. We also calculate the polarization signal for a set of clusters, taken from large-scale structure simulations and evaluate the impact of the gas bulk motions on the polarization signal. We argue that the expected degree of polarization is within reach of the next generation of space X-ray polarimeters.