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

Abstract

The flat spectrum radio quasars (FSRQs) are a sub-class of blazars characterised by prominent optical emission lines and a collimated large-scale jet along the observer line of sight. An X-ray spectral flattening has been reported in FSRQs (at relatively high redshifts), attributable to either absorption from gas along the line of sight or intrinsic jet based radiative processes. We study a sample of 16 high redshift FSRQs (z of 1.1 -- 4.7; rest frame energy upto 50 keV) observed with 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 ten of the sixteen FSRQs. Intrinsic jet based radiative processes are indicated in four 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 in identifying X-ray absorption signatures. A clear detection can be employed to disentangle the relative contributions from the inter-galactic medium and the intra-cluster medium, the methodology of which is outlined and applied to the latter two sources.