Comprehensive view of a z~6.5 radio-loud quasi-stellar object: From the radio to the optical/NIR to the X-ray band

Abstract

We present a multi-wavelength analysis, from the radio to the X-ray band, of the redshift $z=6.44$ VIK J2318$-$31 radio-loud quasi-stellar object, one of the most distant currently known of this class. The work is based on newly obtained observations (uGMRT, ATCA, and Chandra ) as well as dedicated archival observations that have not yet been published (GNIRS and X-shooter). Based on the observed X-ray and radio emission, its relativistic jets are likely young and misaligned from our line of sight. Moreover, we can confirm, with simultaneous observations, the presence of a turnover in the radio spectrum at $ peak 650$ MHz that is unlikely to be associated with self-synchrotron absorption. From the near-infrared spectrum we derived the mass of the central black hole BH odot $, and the Eddington ratio, $ EDD $, using broad emission lines as well as an accretion disc model fit to the continuum emission. Given the high accretion rate, the presence of a sim 8times 10$^8$ M$_ black hole at $z=6.44$ can be explained by a seed black hole (sim 104M$_ that formed at $z assuming a radiative efficiency $ d However, by assuming $ d as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation.