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

Abstract

We report Chandra detection of three UV bright radio quiet quasars at $z\gtrsim 5$. 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 ($\Gamma$), and optical-to-X-ray spectral slope (${\alpha }_{\mathrm{OX}}$). 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 $\sim 3800\mathrm{s}$ in the observer's frame, or $\sim 600\mathrm{s}$ 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 $\sim 2.7$ times of the "low hard" state, and a significantly steeper X-ray spectral slope ($\Gamma =2.40_{-0.32}^{+0.33}$ vs $1.78_{-0.24}^{+0.25}$). We also compare the three quasars detected in this paper to other quasar samples. We find that J074749+115352, with a SMBH mass of $M_{\mathrm{SMBH}}\approx 1.8\times 10^9{\mathrm{M}}_{\odot }$ and an Eddington ratio of ${\lambda }_{\mathrm{Edd}}\approx 2.3$, is extraordinarily X-ray bright. It has an average ${\alpha }_{\mathrm{OX}}=-1.46\pm 0.02$ and a 2-10 keV bolometric correction factor of $L_{\mathrm{bol}}/L_{2-10keV}=42.4\pm 5.8$, both significantly depart from some well defined scaling relations. We compare $\Gamma$ 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.

arXiv comment12 pages, 5 figures, 2 tables, accepted for publication by ApJ

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