X-ray studies

People involved at OAB: Comastri, Vignali

The large collecting area of XMM-Newton has allowed to perform high-quality spectral analysis of the bright narrow-line Seyfert 1 galaxy Ark 564, which was observed twice between 2000 and 2001. The 0.6-10 keV continuum is well described by a soft blackbody component with a temperature of $kT\approx$ 140-150 eV plus a steep power law ($\Gamma\approx$ 2.50-2.55). The source has shown significant X-ray flux variations ($\approx$ 40-50%) over a time scale of one year, despite the lack of spectral changes. In both observations an absorption edge at a rest-frame energy of $\approx$ 0.73 keV, corresponding to OVII, has been detected. The XMM-Newton data support the idea that the power spectral density shows two breaks, although the location of the high-frequency break requires further constraints. Curiously, the doubly broken PSD resembles that seen in Cyg X-1 in its low/hard state, contrary to the expectation that Ark 564 might look more like the high/soft state (which only shows the high-frequency break).

A significant number of the high-redshift ($z>4$) quasars discovered by recent optical surveys have been followed-up in the X-rays. These studies have been enabled primarily by the imaging and spectroscopic capabilities of Chandra and XMM-Newton. From a general perspective, one of the most interesting results is that the X-ray properties of $z>4$ quasars are similar to those of local quasars. Through a joint X-ray spectral fitting of a sample of 46 radio-quiet quasars (RQQs) with Chandra detections in the redshift range 4.0-6.3 ($\approx$ 750 source counts), it has been found that a power law with $\Gamma=1.9\pm{0.1}$ is a good parameterization of the rest-frame $\approx$ 3-40 keV continuum. No evidence for widespread X-ray absorption has been found, although some quasars are likely to be obscured. Overall, the emerging picture is that the small-scale X-ray emission regions of quasars appear relatively insensitive to large-scale environmental differences at $z\approx$ 6. Similarly to the results obtained for the RQQs, neither the X-ray spectral slope ( $\Gamma\approx1.6\pm{0.1}$) nor the jet emission of the radio-loud quasar population seem to evolve significantly with cosmic time. No evidence for significant X-ray brightening ascribed to inverse Compton scattering of energetic electrons with Cosmic Microwave Background photons has been revealed by snapshot observations with Chandra.

The low background, sharp PSF, and sub-arcsec spatial resolution provided by Chandra have also allowed the study of the X-ray properties of 6 objects selected from among the OH gigamaser (OHG) sources of the Darling & Giovanelli (2002) sample. This study was aimed at understanding the nature of the engine powering OHGs. The Chandra snapshot ($\approx$ 4-5 ks) observations have allowed us to place constraints on the relative contribution between the AGN and starburst emission. None of the sources has been detected in the X-ray band; this is consistent with the hypothesis that most of the power in OHGs comes from a starburst, although it is not possible to rule out, at least in some objects, the presence of Compton-thick (i.e., $N_{\rm H}>10^{24}$ cm$^{-2}$) absorption.

The superb imaging capabilities of Chandra have been further exploited to study the complex nuclear and extended emission (lobes and jets) of the high redshift radio galaxy 3C 265. The heavily absorbed nuclear luminosity provides, together with the microwave background radiation, the seed photons which are upscattered by inverse Compton with the relativistic electrons in the lobes in the X-ray band. A detailed analysis of the physical properties of the jets and lobes is ongoing.