Belladitta, S.S.BelladittaMORETTI, AlbertoAlbertoMORETTICACCIANIGA, AlessandroAlessandroCACCIANIGASpingola, C.C.SpingolaSEVERGNINI, PaolaPaolaSEVERGNINIDELLA CECA, RobertoRobertoDELLA CECAGHISELLINI, GabrieleGabrieleGHISELLINIDALLACASA, DANIELEDANIELEDALLACASASbarrato, TulliaTulliaSbarratoCicone, C.C.CiconeCASSARA, LETIZIA PASQUALETIZIA PASQUACASSARAPedani, M.M.Pedani2021-09-202021-09-2020200004-6361http://hdl.handle.net/20.500.12386/31043We 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 $\sim$3.4$\times$10$^{-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 ($\alpha_{\nu r}$<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. %i.e. a radio-loud (RL) AGN with the relativistic jet pointed toward us. 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$\sim$6 and with M$_{1450 \mbox{\AA}}$ < -25.1 of 5.5$^{+11.2}_{-4.6}$$\times$10$^{-3}$ Gpc$^{-3}$. From this number, and assuming a reasonable value of the bulk velocity of the jet ($\Gamma$=10), we can also infer a space density of the entire radio-loud AGN population at z$\sim$6 with the same optical/UV absolute magnitude of 1.10$^{+2.53}_{-0.91}$ Gpc$^{-3}$. Larger samples of blazars will be necessary to better constrain these estimates.STAMPAenArticle10.1051/0004-6361/2019373952-s2.0-85094120427WOS:000519539300002https://www.aanda.org/articles/aa/full_html/2020/03/aa37395-19/aa37395-19.htmlhttp://arxiv.org/abs/2002.05178v2FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_9 Clusters of galaxies and large scale structures