Showing papers by "Andrea Comastri published in 1996"

On the soft X-ray spectra of gamma-loud blazars

Abstract: ROSAT observations of a large sample of bright gamma-ray (E > 100 MeV) blazars are presented. Results of a detailed spectral analysis in the soft $\sim$ 0.1-2.0 keV energy range are discussed in relation to the overall energy distribution with particular emphasis on the relation between X-ray and gamma-ray properties. A significant anti-correlation between X-ray and gamma-ray spectral shapes of flat radio spectrum quasars (FSRQ) and BL Lacs has been discovered. A different shape in the overall energy distributions from radio to gamma-ray energies between FSRQ and BL Lacs is also implied by the correlation of their broad-band spectral indices $\alpha_{ro}$ and $\alpha_{x \gamma}$. Both the above correlations can be explained if both the IR to UV emission and the hard X-ray to gamma-ray emission originate from the same electron population, via, respectively, the synchrotron process and the inverse Compton mechanism. We suggest that a key parameter for understanding the overall energy distributions of both classes of objects is the energy at which the synchrotron emission peaks in a $ u- u F( u)$ representation.

The Sources of the Hard X-ray Background

Abstract: The hard component (3 keV – ~ MeV) of the X-ray background (XRB) comprises the largest portion, ~ 90%, of the overall XRB intensity. The observed isotropy (the entire Galaxy is transparent above 3 keV) provides a prima facie evidence of its prevailing extragalactic nature. A large fraction (~ 75%) of the energy flux falls in the 3–100 keV band, the corresponding energy density being ≃ 5x10-5 eV cm-3, of which 50% is confined to the narrower 3–20 keV band. Although the energy flux carried by the XRB is relatively small compared to other extragalactic backgrounds, it was soon realized that it cannot be accounted for in terms of sources and processes confined to the present epoch. An analysis of the combined observed spectra (Gruber 1992) concludes that, while a thermal bremsstrahlung with an efolding energy = 41.13 keV accurately fits the data up to 60 keV, above this energy the sum of two power laws is requuired with normalizations such that at 60 keV the spectral index is ~ 1.6, gradually flattening to ~ 0.7 at MeV energies. It should also be noted that below 10 keV the XRB energy spectrum is well represented by a power law of index α = 0.4 (I ∝ E-α)

ASCA and ROSAT X-ray Spectra of High-Redshift Radio-Loud Quasars

Abstract: Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by ASCA (10 observations) and ROSAT (11 observations, for a subset of 6 quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere, are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the 6 brightest objects show evidence for absorption in excess of the Galactic value at a << 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied ($\Delta N_{H} \sim 8$ $\times$ 10$^{20}$ cm$^{-2}$) in the case of S5 0836+71, on a time-scale of $\sim$ 0.8 yr in the quasar frame. For the remaining 5 sources for which ROSAT spectra were available, the two instruments gave consistent results and the data were combined yielding unprecedent spectral coverage (typically $\sim$ 0.4-40 keV in the quasar frame) for high-z quasars. This allows to put severe limits on several different descriptions of the continuum (e.g. broken power law, bremsstrahlung, reflection component). No Fe K$\alpha$ emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe K$\alpha$ at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In the light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average

On the X-Ray Emission from the Powerful Radio Galaxies

Abstract: There is a growing evidence that radio loud quasars and powerful FR II radio galaxies belong to the same population. While X-ray observations of low redshift radio galaxies [1], [2] generally support the unified scheme relating the FR II radio galaxies to the radio quasars, nevertheless detailed studies of the X-ray properties of distant radio galaxies are made difficult due to both the low sensitivity of X-ray-satellites and to the emission of the hot intracluster gas in which they are normally embedded. We point out that significant fluxes of X-rays are produced in the strong radio galaxies by the Inverse Compton (IC) process. In the framework of the unified scheme the radio galaxies are pervaded both by an intense radiation flux from the misdirected hidden quasar and the cosmic microwave background radiation flux (CMB). From the standpoint of IC computation the far and near-IR emissions of the hidden quasar are of particular importance.