Chandra Detection of Three X-Ray Bright Quasars at z > 5

Abstract

We report Chandra detection of three UV-bright radio-quiet quasars at z ≳5. We have collected a sufficient number of photons to extract an X-ray spectrum of each quasar to measure their basic X-ray properties, such as the X-ray flux, power-law photon index (Γ), and optical-to-X-ray spectral slope (αOX). J074749+115352 at z = 5.26 is the X-ray brightest radio-quiet quasar at z > 5. It may have a short timescale variation (on a timescale of ∼3800 s in the observer's frame, or ∼600 s in the rest frame), which is, however, largely embedded in the statistical noise. We extract phase folded spectra of this quasar. There are two distinguishable states: A "high soft"state with an average X-ray flux ∼2.7 times the "low hard"state, and a significantly steeper X-ray spectral slope (Γ = - 2.40+0.32 0.33 versus - 1.78+0.24 0.25). We also compare the three quasars detected in this paper to other quasar samples. We find that J074749+115352, with an SMBH mass of MSMBH ≈ 1.8 × 109Me and an Eddington ratio of λEdd ≈ 2.3, is extraordinarily X-ray bright. It has an average αOX = -1.46 ± 0.02 and a 2-10 keV bolometric correction factor of Lbol/L2-10 keV = 42.4 ± 5.8, both significantly depart from some well defined scaling relations. We compare Γ of the three quasars to other samples at different redshifts, and do not find any significant redshift evolution based on the limited sample of z > 5 quasars with reliable measurements of the X-ray spectral properties. © 2021. The American Astronomical Society.