We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) 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 S wi f t Observatory XRT observation allowed us to measure a flux of ∼3.4×10 −14 erg s −1 cm −2 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>10 3 ), 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 −4.6 ×10 −3 Gpc −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 −0.91 Gpc −3 . Larger samples of blazars will be necessary to better constrain these estimates.

We present Director's Discretionary Time multi-frequency observations obtained with the Jansky Very Large Array and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at z = 6.10 ± 0.03. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5, and 8.4 GHz unveils a bright one-sided jet extended for ∼500 parsecs in projection. This high-z radio-loud active galactic nucleus is resolved into multiple compact sub-components that are embedded in a more diffuse and faint radio emission that enshrouds them in a continuous jet structure. We directly derive limits on some physical parameters from observable quantities such as viewing angle and Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (lower than 5). This value would be in favour of a scenario currently proposed to reconcile the paucity of high-z blazars with current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton scattering with the cosmic microwave background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and on these factors in the other high-z blazars are necessary to test whether their properties are intrinsically different from those of the low-z blazar population.

We report the 888 MHz radio detection in the Rapid ASKAP Continuum Survey (RACS) of VIK J2318−3113, a z = 6.44 quasar. Its radio luminosity (1.2 × 1026 W Hz−1 at 5 GHz) compared to the optical luminosity (1.8 × 1024 W Hz−1 at 4400 Å) makes it the most distant radio-loud quasar observed so far, with a radio loudness R ∼ 70 (R = L5 GHz/L4400 Å). Moreover, the high bolometric luminosity of the source (Lbol = 7.4 × 1046 erg s−1) suggests the presence of a supermassive black hole with a high mass (≳6 × 108 M⊙) at a time when the Universe was younger than a billion years. Combining the new radio data from RACS with previous ASKAP observations at the same frequency, we found that the flux density of the source may have varied by a factor of ∼2, which could suggest the presence of a relativistic jet oriented towards the line of sight, that is, a blazar nature. However, currently available radio data do not allow us to firmly characterise the orientation of the source. Further radio and X-ray observations are needed.