Low‐Energy Cutoffs and Hard X‐Ray Spectra in High‐zRadio‐loud Quasars: TheSuzakuView of RBS 315

Abstract

We present the results from the Suzaku observation of the powerful radio-loud quasar RBS 315 (z = 2.69), for which a previous XMM-Newton observation showed an extremely flat X-ray continuum up to 10 keV (photon index Γ = 1.26) and indications of strong intrinsic absorption (NH ~ 1022 cm-2, assuming neutral gas). The instrument for hard X-rays, HXD/PIN, allows us a detection of the source up to 50 keV. The broadband continuum (0.5-50 keV) can be well modeled with a power law with slope Γ = 1.5 (definitively softer than the continuum measured by XMM-Newton) above 1 keV with strong deficit of soft photons. The low-energy cutoff can be well fitted, either with intrinsic absorption (with column density NH ~ 1022 cm-2 in the quasar rest frame) or with a break in the continuum, with an extremely hard (Γ = 0.7) power law below 1 keV. We construct the spectral energy distribution of the source, using also optical-UV measurements obtained through a quasi-simultaneous observation with UVOT on board Swift observation. The shape of the SED is similar to that of other flat-spectrum radio quasars (FSRQs) with similar power, making this source an excellent candidate for the detection in γ-rays by the Gamma-Ray Large Area Space Telescope (GLAST). We model the SED with the synchrotron inverse Compton model usually applied to FSRQs, showing that the deficit of soft photons can be naturally interpreted as due to an intrinsic curvature of the spectrum near the low-energy end of the IC component, rather than to intrinsic absorption, although the latter possibility cannot be ruled out. We propose that in at least a fraction of the radio-loud QSOs at high redshift, the cutoff in the soft X-ray band can be explained in a similar way. Further studies are required to distinguish between the two alternatives.