2021ApJ...906..135L

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 (α_OX). 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 M_SMBH ≃ 1.8 x 10⁹ M_☉ and an Eddington ratio of λ_Edd ≃ 2.3, is extraordinarily X-ray bright. It has an average α_OX = -1.46 ± 0.02 and a 2-10 keV bolometric correction factor of L_bol/L_2-10 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.