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The central engine of the highest redshift blazar

We report on a LUCI/Large Binocular Telescope near-infrared (NIR) spectrum of PSO J030947.49+271757.31 (hereafter PSO J0309+27), the highest redshift blazar known to date (z$\sim$6.1). From the C$\rm IV$$λ$1549 broad emission line we found that PSO J0309+27 is powered by a 1.45$^{+1.89}_{-0.85}$$\times$10$^9$M$_{\odot}$ supermassive black hole (SMBH) with a bolometric luminosity of $\sim$8$\times$10$^{46}$ erg s$^{-1}$ and an Eddington ratio equal to 0.44$^{+0.78}_{-0.35}$. We also obtained new photometric observations with the Telescopio Nazionale Galileo in J and K bands to better constrain the NIR Spectral Energy Distribution of the source. Thanks to these observations, we were able to model the accretion disk and to derive an independent estimate of the black hole mass of PSO J0309+27, confirming the value inferred from the virial technique. The existence of such a massive SMBH just $\sim$900 million years after the Big Bang challenges models of the earliest SMBH growth, especially if jetted Active Galactic Nuclei are associated to a highly spinning black hole as currently thought. Indeed, in a Eddington-limited accretion scenario and assuming a radiative efficiency of 0.3, typical of a fast rotating SMBH, a seed black hole of more than 10$^6$ M$_{\odot}$ at z = 30 is required to reproduce the mass of PSO J0309+27 at redshift 6. This requirement suggests either earlier periods of rapid black hole growth with super-Eddington accretion and/or that only part of the released gravitational energy goes to heat the accretion disk and feed the black hole.