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 γ -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 Nuclear Spectroscopic Telescope Array , Swift,  and the Southeastern Association for Research in Astronomy’s optical telescopes. We observe six high-redshift quasars, four of them belonging to the new γ -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 rehardening of the X-ray spectrum at energies E  > 6 keV. Adopting a one-zone leptonic emission model, this combination of hard X-rays and γ -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 black holes, further raising the space density of supermassive black holes in the redshift bin z  = [3, 4].