Breaks in the X-ray spectra of high-redshift blazars and the intervening medium

Abstract

Flat-spectrum radio quasars (FSRQs) are a subclass of blazars characterized by prominent optical emission lines and a collimated large-scale jet along the observer's line of sight. An X-ray spectral flattening has been reported in FSRQs (at relatively high redshifts) that is attributable either to absorption from gas along the line of sight or to intrinsic jet-based radiative processes. We study a sample of 16 high-redshift FSRQs (z of 1.1–4.7; rest-frame energy up to 50 keV) observed with the XMM-Newton and Swift satellites spanning 29 epochs. The X-ray spectra are fit with a power law including free excess absorption and one multiplied by an exponential roll-off to account for the intrinsic jet-based processes. A statistical analysis is used to distinguish between these models to understand the origin of the spectral flattening. The model selection is unable to distinguish between them in 10 of the 16 FSRQs. Intrinsic jet-based radiative processes are indicated in 4 FSRQs, where we infer energy breaks consistent with their expectation from the external Compton-scattering of low-energy ambient photons. Two of the FSRQs indicate mixed results, supportive of either scenario, illustrating the difficulty of identifying X-ray absorption signatures. A clear detection can be employed to disentangle the relative contributions from the intergalactic medium and the intracluster medium, the method of which is outlined and applied to the latter two sources.