SED Constraints on the Highest-z Blazar Jet: QSO J0906+6930

We report on Gemini, NuSTAR, and eight years of 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 synchroCompton model. The measurements yield a mass of similar to 4 x 10(9) M-circle dot for the black hole and a spectral break at similar to 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(-3)(+2.5) for QSO J0906+6930. Similar, but weaker, constraints on d are derived from SED modeling of the three other claimed z > 5 blazars. Together, these extrapolate to similar to 620 similar sources, fully 20% of the optically bright, high-mass active galactic nuclei expected at 5 < z < 5.5. This has interesting implications for the early growth of massive black holes.