142 high-redshift blazars at the cosmic dawn

In an attempt to identify the most extreme members of the quasar population, that is, blazars, we have carried out a multiwavelength study of a large sample of radio-loud quasars beyond z=3. Our sample consists of nine {gamma}-ray-detected blazars and 133 candidate blazars selected based on the flatness of their soft X-ray spectra (0.3-10keV photon index <~1.75), including 15 with Nuclear Spectroscopic Telescope Array (NuSTAR) observations.

The application of the likelihood profile stacking technique reveals that the high-redshift blazars are faint {gamma}-ray emitters with steep spectra. The high-redshift blazars host massive black holes (<logM_BH,M{odot}_>>9) and luminous accretion disks (<L_disk_>>10^46^erg/s). Their broadband spectral energy distributions are found to be dominated by high-energy radiation, indicating their jets are among the most luminous ones.