NuSTAR perspective on high-redshift MeV blazars

Abstract

With bolometric luminosities exceeding $10^{48}$ erg s${}^{-1}$, powerful jets and supermassive black holes at their center, MeV blazars are some of the most extreme sources in the Universe. Recently, the Fermi-Large Area Telescope detected five new ${\hat{I}}^3$-ray emitting MeV blazars beyond redshift $z=3.1$. With the goal of precisely characterizing the jet properties of these extreme sources, we started a multiwavelength campaign to follow them up with joint NuSTAR, Swift and SARA observations. We observe six high-redshift quasars, four of them belonging to the new ${\hat{I}}^3$-ray emitting MeV blazars. Thorough X-ray analysis reveals spectral flattening at soft X-ray for three of these objects. The source NVSS J151002$+$570243 also shows a peculiar re-hardening of the X-ray spectrum at energies $E>6\mathrm{keV}$. Adopting a one-zone leptonic emission model, this combination of hard X-rays and ${\hat{I}}^3$-rays enables us to determine the location of the Inverse Compton peak and to accurately constrain the jet characteristics. In the context of the jet-accretion disk connection, we find that all six sources have jet powers exceeding accretion disk luminosity, seemingly validating this positive correlation even beyond $z>3$. Our six sources are found to have $10^9{\mathrm{M}}_{\odot }$ black holes, further raising the space density of supermassive black holes in the redshift bin $z=[3,4]$.

17 pages, 7 figures, 8 tables, 1 appendix, accepted for publication in ApJ