The space density of z>4 blazars

High redshift blazars are an important class of Active Galactic Nuclei (AGN) that can provide an independent estimate of the supermassive black-hole mass function in high redshift radio-loud AGN without the bias due to absorption along the line-of-sight. Using the Cosmic Lens All Sky Survey (CLASS) we built a complete radio flux-limited sample of high redshift (z>4) blazars suitable for statistical studies. By combining dedicated optical observations and the SDSS spectroscopic database, we obtained a sample of 26 blazar candidates with a spectroscopic redshift above 4. On the basis of their radio spectrum we distinguish between blazars and QSO with a Gigahertz Peaked Spectrum (GPS) like spectrum. Out of the 18 confirmed blazars 14 constitute a completely identified, flux-limited sample down to a magnitude of 21 (AB). Using this complete sample we derive a space density of blazars with 4<z<5.5 of rho=0.13 (+0.05,-0.03) Gpc^-3. This is the first actual estimate of the blazar space density in this range of redshift. This value is in good agreement with the extrapolation of the luminosity function and cosmological evolution based on a sample of flat-spectrum radio quasars selected at lower redshifts and it is consistent with a cosmological evolution peaking at z$\sim$2 similar to radio-quiet QSO. We do not confirm, instead, the presence of a peak at z~4 in the space density evolution, recently suggested using an X-ray selected sample of blazars. It is possible that this extreme peak of the evolution is present only among the most luminous blazars.Comment: 14 pages, accepted for publication on MNRAS (https://doi.org/10.1093/mnras/sty3526

Minute-timescale Variability in the X-ray Emission of the Highest Redshift Blazar

We report on two Chandra observations of the quasar PSO J0309+27, the most distant blazar observed so far (z = 6.1), performed eight months apart, in 2020 March and November. Previous Swift-XRT observations showed that this object is one of the brightest X-ray sources beyond redshift 6.0 ever observed so far. This new dataset confirmed the high flux level and unveiled a spectral change that occurred on a very short timescale (250 s rest frame), caused by a significant softening of the emission spectrum. This kind of spectral variability, on such a short interval, has never been reported in the X-ray emission of a flat-spectrum radio quasar. A possible explanation for this is given by the emission produced by the inverse Compton scatter of the quasar UV photons by the cold electrons present in a fast shell moving along the jet. Although this bulk Comptonization emission should be an unavoidable consequence of the standard leptonic jet model, this would be the first time that it has been observed

Blazars in the re-ionization epoch

Search and Study of Blazars in the early Universe PhD thesis by Silvia Belladitta , Matr. 733846 The aim of this thesis is to identify and characterize blazars at z&gt;4, to provide more constraints on the properties of high-z radio-loud (RL) Active Galactic Nuclei (AGNs) population. Blazars are RL AGNs with relativistic jets pointing towards the Earth. The discovery of high-z blazars ensures the census, free from obscuration effects, of supermassive black holes (SMBHs) hosted by RL AGNs in the early Universe. Indeed for each observed blazar we expect to find ~2\u3932 (where \u393 is the bulk Lorentz factor of the jet) RL AGNs, whose jets point in other directions, with the same properties (e.g. MBH, Lradio, etc.). By combining different radio/optical/infrared surveys available to date and by using the dropout method I built up a statistically complete sample (21 objects) of blazars at 4&lt;5.5, and I discoverer new (3) blazars at z&gt;5. One of these, at z=6.1, is also the most distant blazar ever discovered. With these sources I increased by 50% the number of blazars known to date at z&gt;4. I performed a multi wavelength analysis of the properties of all these sources (from radio to X-ray bands) and I compared them to both RL and radio-quiet (RQ, AGNs without relativistic jets) AGNs at similar redshift. The blazars reported in this thesis are all powerful X-ray sources, have a similar radio spectrum (flat or peaked) and host SMBHs with MBH&gt;108M 99. The presence of such massive objects at high-z requires possible super-eddington accretion episodes for the seed black holes growth

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$$\lambda$1549 broad emission line we found that PSO J0309+27 is powered by a 1.45$^{+1.89}_{-0.85}$$\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.Comment: 10 pages, 5 figures, 2 tables; Accepted to publication in A&