The first blazar observed at z > 6

Abstract

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 Swift 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 (ανr  <   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 M1450 Å