The first blazar observed at z>6. (arXiv:2002.05178v1 [astro-ph.CO])

The first blazar observed at z>6. (arXiv:2002.05178v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Belladitta_S/0/1/0/all/0/1">S. Belladitta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moretti_A/0/1/0/all/0/1">A. Moretti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Caccianiga_A/0/1/0/all/0/1">A. Caccianiga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Spingola_C/0/1/0/all/0/1">C. Spingola</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Severgnini_P/0/1/0/all/0/1">P. Severgnini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ceca_R/0/1/0/all/0/1">R. Della Ceca</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ghisellini_G/0/1/0/all/0/1">G. Ghisellini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dallacasa_D/0/1/0/all/0/1">D. Dallacasa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sbarrato_T/0/1/0/all/0/1">T. Sbarrato</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cicone_C/0/1/0/all/0/1">C. Cicone</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cassara_L/0/1/0/all/0/1">L. P. Cassar&#xe0;</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pedani_M/0/1/0/all/0/1">M. Pedani</a>

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 $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}}$ < -21.5 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.

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 $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}}$ < -21.5 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.

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