X-ray properties of z>4 blazars

TL;DR

This study analyzes the X-ray properties of a complete sample of high-redshift (z>4) blazars, revealing an increased X-ray-to-radio luminosity ratio compared to lower-redshift blazars, possibly due to CMB interactions, but not fully explaining evolutionary differences.

Contribution

First statistical analysis of X-ray properties of a complete, flux-limited z>4 blazar sample, highlighting potential CMB effects on high-redshift jet emissions.

Findings

High-z blazars have 2.4 times higher X-ray-to-radio luminosity ratio than low-z counterparts.

Evidence suggests CMB interactions may boost X-ray emission at high redshift.

The simple CMB interaction model does not fully resolve blazar evolutionary discrepancies.

Abstract

We present the X-ray analysis of the largest flux-limited complete sample of blazar candidates at z>4 selected from the Cosmic Lens All Sky Survey (CLASS). After obtaining a nearly complete (24/25) X-ray coverage of the sample (from Swift-XRT, XMM-Newton and Chandra), we analysed the spectra in order to identify the bona-fide blazars. We classified the sources based on the shape of their Spectral Energy Distributions (SEDs) and, in particular, on the flatness of the X-ray emission and its intensity compared to the optical one. We then compared these high-z blazars with a blazar sample selected at lower redshifts (z~1). We found a significant difference in the X-ray-to-radio luminosity ratios, with the CLASS blazars having a mean ratio 2.4+/-0.5 times larger than low-z blazars. We tentatively interpret this evolution as due to the interaction of the electrons of the jet with the Cosmic Microwave Background (CMB) photons, which is expected to boost the observed X-ray emission at high redshifts. Such a dependence has been already observed in highly radio-loud AGNs in the recent literature. This is the first time it is observed using a statistically complete radio flux limited sample of blazars. We have then evaluated whether this effect could explain the differences in the cosmological evolution recently found between radio and X-ray selected samples of blazars. We found that the simple version of this model is not able to solve the tension between the two evolutionary results.