2020A&A...635L...7B

We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7mJy at 1.4GHz) active galactic nucleus (AGN) at z>6.0. It was selected by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-z nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed Neil Gehrels Swift Observatory XRT observation allowed us to measure a flux of ∼3.4x10^–14erg/s/cm² in the [0.5-10]keV energy band, which also makes this object the X-ray brightest AGN ever observed at z>6.0. Its flat radio spectrum (α_νr<0.5), very high radio loudness (R>10³), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z∼6 and with M_1450Å←25.1 of 5.5^+11.2_–4.6x10^–32x10^–3Gpc^–3. From this number, and assuming a reasonable value of the bulk velocity of the jet (Γ=10), we can also infer a space density of the entire radio-loud AGN population at z∼6 with the same optical/UV absolute magnitude of 1.10^+2.53_–0.91Gpc^–3. Larger samples of blazars will be necessary to better constrain these estimates.