ASCAandROSATX‐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 (∆N H ∼ 8 × 10 20 cm −2 ) in the case of S5 0836+71, on a time-scale of ∼ 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 ∼ 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 Kα emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe Kα 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 slope obtained from the 8 ASCA spectra in the observed ∼ 0.5-10 keV energy band is < Γ 0.5−10keV > ≃ 1.61 ± 0.04, with a dispersion σ 0.5−10keV ≃ 0.10 ± 0.03. The average photon index in the observed 2-10 keV band, where the effect of absorption is negligible, is < Γ 2−10keV > ≃ 1.53 ± 0.05, with a dispersion σ 2−10keV < ∼ 0.12. Furthermore, the implications of the present results on the calculations of the contribution of quasars to the cosmic X-and γ-ray backgrounds (XRB and GRB) are briefly discussed.

Ultraluminous X-ray source XMMUJ132218.3-164247 is in fact a type I Quasar

“…EW: emission line equivalent width. Finally, the X-ray spectrum of the source is flat (Γ ∼ 1.6) comparing with nearby AGNs (Γ ∼ 1.8−2; Dadina 2008) but, when the errors are considered, it is consistent with what have been found for high-z quasars (Γ = 1.6 ± 0.4 Cappi et al 1997; Γ = 1.86 +0.41 −0.37 Vignali et al 2003). This value, however, is not consistent with the one previously measured by Miniutti et al (2006) at a time when the source was weaker and displayed a steeper spectrum (F 0.5−7 keV ∼ 8 × 10 −14 erg s −1 cm −2 , while in 2008 F 0.5−7 keV ∼ 1.2×10 −13 erg s −1 cm −2 and Γ ∼ 1.95 in 2006).…”

The changing look of PKS 2149-306

“…As for other high-z FSRQs, a controversial low-energy photon deficit has been claimed by some authors but not confirmed by others (Siebert et al 1996;Cappi et al 1997;Yaqoob et al 1999;Sambruna et al 2007;Elvis et al 2000;Ferrero & Brinkmann 2003;Page et al 2005). This putative deficit was usually ascribed to an absorbing cloud in the quasar reference frame and modeled by additional intrinsic absorption in addition to the Galactic one.…”

The X-ray to optical-UV luminosity ratio of X-ray selected type 1 AGN in XMM-COSMOS

“…It is well known that radio-loud AGN have an enhanced X-ray emission mechanism linked to the jets, which can provide an increment in the X-ray emission with respect to radio-quiet AGN with similar optical luminosities (e.g., Zamorani et al 1981;Wilkes & Elvis 1987;Cappi et al 1997). We exclude RL AGN from the total sample using the "standard" definition of radio-loudness, R ≥ 10 ( Kellermann et al 1989) 4 .…”