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

TL;DR

The study addresses how X-ray emission from the most luminous quasars at $z>6.5$ tracks accretion and winds, and how their X-ray properties relate to rest-frame UV/optical and infrared characteristics. By assembling seven new Chandra observations and combining them with archival data, the authors construct a nearly complete X-ray census of $z\sim7$ quasars with $M_{1450}<-26.5$, enabling a joint analysis of $L_{\rm X}$, $\alpha_{\rm OX}$, $L_{2500}$, $L_{bol}$, CIV blueshift, and $L_{IR}$. They find that $z>6.5$ quasars largely follow the established $\alpha_{\rm OX}-L_{\mathrm{2500\AA}}$ relation but with large scatter, observe a potential $\alpha_{\rm OX}$–CIV blueshift link suggesting soft optical/UV-to-X-ray SEDs accompany fast disk winds, and measure a best-fit X-ray photon index $\Gamma=2.41\pm0.27$ indicative of high accretion rates. Additionally, there are no strong correlations between $L_{\rm X}$ and $L_{IR}$ nor between $L_{bol}$ and $L_{IR}$, implying a weak connection between quasar luminosity and host galaxy star formation for these extreme objects. Overall, the work tightens constraints on accretion physics and wind phenomena in the early universe and informs models of SMBH growth.

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 $α_{\rm OX}$) 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 $α_{\rm OX}-L_{\mathrm{2500\ \mathring{A}}}$ relation as $z\sim1-6$ quasars, but also display a large scatter. We find a potential correlation between $α_{\rm OX}$ 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 $Γ$ 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 $z>6.5$.