A Deep Chandra X-ray Survey of a Luminous Quasar Sample at $z\sim$ 7

TL;DR

This study presents comprehensive X-ray observations of luminous quasars at redshift around 7, revealing their spectral properties, correlations with other quasar features, and insights into their accretion and wind mechanisms.

Contribution

It provides the first nearly complete X-ray survey of all known luminous quasars at z~7, analyzing their spectral slopes, correlations with wind indicators, and accretion rates.

Findings

Most z>6.5 quasars follow the alpha_OX-L_2500A relation similar to lower redshift quasars.

A potential correlation exists between alpha_OX and C IV blueshift, indicating link to disk winds.

The X-ray photon index is high (~2.41), likely due to high accretion rates.

Abstract

We present new Chandra observations of seven luminous quasars at $z>6.5$. Combined with archival Chandra observations of all other known quasars, they form nearly complete X-ray observations of all currently known $z\sim7$ quasars with $M_{1450}<-26.5$, except for J0313$-$1806 at $z=7.642$ and J0910$-$0414 at $z=6.636$. Together with existing ground-based NIR spectroscopy and ALMA observations, we investigate the correlations between X-ray emission (the X-ray luminosity $L_{\rm X}$ and the optical/UV-to-X-ray spectral slope $\alpha_{\rm OX}$) and various quasar properties (rest-UV luminosity $L_{\mathrm{2500\ \mathring{A}}}$, bolometric luminosity $L_{\rm bol}$, C IV blueshift, and infrared luminosity $L_{\rm IR}$). We find most $z>6.5$ quasars follow a similar $\alpha_{\rm OX}-L_{\mathrm{2500\ \mathring{A}}}$ relation as $z\sim1-6$ quasars, but also display a large scatter. We find a potential correlation between $\alpha_{\rm OX}$ and the C IV blueshift, suggesting a soft optical/UV-to-X-ray SED shape is frequently associated with fast disk winds. Furthermore, we analyze the X-ray spectrum of 11 quasars at $z>6.5$ with Chandra detection, and find the best-fit photon index $\Gamma$ is $2.41\pm0.27$, which is likely driven by high accretion rates of $z>6.5$ quasars. In addition, we find there are no significant correlations between either $L_{\rm X}$ and $L_{\rm IR}$, nor $L_{\rm bol}$ and $L_{\rm IR}$, suggesting no strong correlations between quasar luminosity and star formation luminosity for the most luminous quasars at $z>6.5$.