arXiv subjects

A. Comastri

Publications and source records attributed to A. Comastri.

345 records · Page 20Linked to original sources

arXiv1995

The X-ray spectrum of the Gamma Bright Quasar S5 0836+710

We present the results of an ASCA observation of the high redshift flat spectrum radio quasar S5 0836+710. The $\sim 0.4-10$ keV X-ray spectrum is remarkably flat with $Γ\sim 1.4$ and substantial intrinsic absorption at low energy. The spectral slope was found to be in good agreement with the non--simultaneous ROSAT PSPC observations while no evidence of intrinsic absorption has been found in the PSPC spectrum. Our results suggest time variability of the absorbing material on timescales less than five months in the source rest--frame.

astro-ph↗

arXiv1994

The contribution of AGNs to the X-ray background

We report the results of a detailed analysis of the contribution of various classes of AGNs to the extragalactic X-ray background (XRB). The model is based on the unification schemes of AGNs and on their related X-ray spectral properties in the light of recent observational results. The integrated emission from AGNs, when folded with an appropriate cosmological evolution law, can provide a good fit to the XRB over a wide energy range, from several to $\sim$ 100 keV, while it contributes only about 74\% of the ROSAT soft XRB. The model predictions have been checked against all available observational constraints from both hard and soft X-ray surveys (counts, redshift distributions and average X-ray source spectral properties).

astro-ph↗

arXiv1994

The variable ROSAT X-ray spectrum of the BL Lac 0716+714

The BL Lac object 0716+714 is a powerful flat spectrum radio source detected by CGRO-EGRET at $γ$-ray energies. It has been observed for about 21000 s over two days by the X-ray telescope onboard the ROSAT satellite and strong flux and spectral variations have been detected. In the time scale of two days the source varied by a factor of $\sim 7$; during a sudden ($\sim 1000$ s) flux increase, the variation was 70\%. 0716+714 exhibits spectral variations related to source flux variability which may be attributed to a ``soft excess" component most prominent when the source was in a low state. The spectrum of 0716+714 steepened in the soft X-ray band when the flux decreased while in the hard X-ray band remained constant. A double power law model, representing steep synchrotron radiation at low energies and flat inverse Compton radiation above $\sim$ 1 keV, best fits both the low and high state.

astro-ph↗