X-ray properties of z>4 blazars
  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.
