SED constraints on the highest-$z$ blazar jet: QSO J0906+6930

Abstract

We report on Gemini, NuSTAR and 8-year Fermi observations of the most distant blazar QSO~J0906$+$6930 ($z=5.48$). We construct a broadband spectral energy distribution (SED) and model the SED using a synchro-Compton model. The measurements find a $\sim 4\times 10^9{M}_{\odot }$ mass for the black hole and a spectral break at $\sim$4 keV in the combined fit of the new NuSTAR and archival Chandra data. The SED fitting constrains the bulk Doppler factor $\hat{I}´$ of the jet to $9_{-3}^{+2.5}$ for QSO~J0906$+$6930. Similar, but weaker $\hat{I}´$ constraints are derived from SED modeling of the three other claimed $z>5$ blazars. Together, these extrapolate to $\sim 620$ similar sources, fully 20\% of the optically bright, high mass AGN expected at $5<z<5.5$. This has interesting implications for the early growth of massive black holes.

10 pages, 5 figures, 2 tables. Accepted for publication in ApJ