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

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 $\delta$ of the jet to $9^{+2.5}_{-3}$ for QSO~J0906$+$6930. Similar, but weaker $\delta$ constraints are derived from SED modeling of the three other claimed $z>5$ blazars. Together, these extrapolate to $\sim620$ 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.