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

TL;DR

This paper reports the detection and analysis of three high-redshift quasars using Chandra, revealing their X-ray properties, variability, and comparison to lower-redshift samples, highlighting the unique brightness of one quasar at z>5.

Contribution

First X-ray spectral analysis of three z>5 quasars, including variability and comparison to lower-redshift populations, expanding understanding of early universe quasars.

Findings

J074749+115352 is the X-ray brightest radio-quiet quasar at z>5.

Detected short timescale variability in one quasar, with distinct spectral states.

One quasar shows significantly different X-ray properties from established scaling relations.

Abstract

We report Chandra detection of three UV bright radio quiet quasars at $z\gtrsim5$. 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 ($\alpha_{\rm OX}$). 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 $\sim3800\rm~s$ in the observer's frame, or $\sim600\rm~s$ 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 $\sim2.7$ 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 $M_{\rm SMBH}\approx1.8\times10^9\rm~M_\odot$ and an Eddington ratio of $\lambda_{\rm Edd}\approx2.3$, is extraordinarily X-ray bright. It has an average $\alpha_{\rm OX}=-1.46\pm0.02$ and a 2-10 keV bolometric correction factor of $L_{\rm bol}/L_{\rm2-10keV}=42.4\pm5.8$, 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.