摘要：We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) atz > 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-znature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointedNeil Gehrels SwiftObservatory XRT observation allowed us to measure a flux of ∼3.4 × 10−14erg s−1cm−2in the [0.5–10] keV energy band, which also makes this object the X-ray brightest AGN ever observed atz > 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 atz ∼ 6 and withM1450 Å< −25.1 of 5.5+11.2−4.6× 10−3Gpc−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 atz ∼ 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.
