2022A&A...663A.147S

We report on the Swift/XRT observation and classification of eleven blazar candidates at z > 4. These sources were selected as part of a sample of extremely radio-loud quasars, with a focus on quasars with jets oriented roughly close to our line of sight. Deriving their viewing angles and their jets' bulk Lorentz factors was crucial for a strict blazar classification, which was made possible only thanks to X-ray observations. Out of eleven sources, five show strong and hard X-ray fluxes that set the foundation for their blazar classification, while two are uncertain and three host relativistic jets that we observe just outside their beaming cone (i.e. are not strictly blazars), while one went undetected by Swift/XRT. Following this approach, we were able to trace the > 10⁹ M_☉ active super-massive black hole (SMBH) population hosted in jetted active galactic nuclei (AGN). At z ≳ 4, the massive jetted sources are likely predominant in the overall quasar population: this calls for a deep review of our understanding of the first SMBH formations and evolution. Jets are indeed key actors in fast accretion and must be searched for across the whole high-redshift quasar population. A note of caution must be added: radio-loudness and, in general, radio features at high redshifts seem do not seem to perfectly reflect high-energy properties. A strong effect attributed to the interaction with cosmic microwave background (CMB) radiation is surely at work, which quenches the radio emission with respect to the X-rays; however, in addition, more frequent occasions for the jet to be bent seem to play a relevant role in this regard. Thus, classifications and population studies must be carefully performed, so as to avoid interference resulting from these inconsistencies.