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

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 $z\sim 7$ 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 $L_{\mathrm{X}}$ and the optical/UV-to-X-ray spectral slope ${\alpha }_{\mathrm{OX}}$) and various quasar properties (rest-UV luminosity $L_{2500\mathring{\mathrm{A}}}$, bolometric luminosity $L_{\mathrm{bol}}$, C IV blueshift, and infrared luminosity $L_{\mathrm{IR}}$). We find most $z>6.5$ quasars follow a similar ${\alpha }_{\mathrm{OX}}-L_{2500\mathring{\mathrm{A}}}$ relation as $z\sim 1-6$ quasars, but also display a large scatter. We find a potential correlation between ${\alpha }_{\mathrm{OX}}$ 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 $2.41\pm 0.27$, which is likely driven by high accretion rates of $z>6.5$ quasars. In addition, we find there are no significant correlations between either $L_{\mathrm{X}}$ and $L_{\mathrm{IR}}$, nor $L_{\mathrm{bol}}$ and $L_{\mathrm{IR}}$, suggesting no strong correlations between quasar luminosity and star formation luminosity for the most luminous quasars at $z>6.5$.

17 pages, 8 figures, 3 tables, accepted for publication in ApJ