SIMBAD references

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 ∼ 7 quasars with M₁₄₅₀ < -26.5, except for J0313-1806 at z = 7.642 and J0910-0414 at z = 6.636. Together with existing ground-based near-infrared spectroscopy and Atacama Large Millimeter Array observations, we investigate the correlations between X-ray emission (the X-ray luminosity L_X and the optical/UV-to-X-ray spectral slope α_OX) and various quasar properties (rest-UV luminosity L_2500 Å_, bolometric luminosity L_bol, C IV blueshift, and infrared luminosity L_IR). We find most z > 6.5 quasars follow a similar α_OX-L_2500 Å_ relation as z ∼ 1-6 quasars, but also display a large scatter. We find a potential correlation between α_OX and the C IV blueshift, suggesting a soft optical/UV-to-X-ray spectral energy distribution 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 that the best-fit photon index, Γ, is 2.41 ± 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_X and L_IR, nor L_bol and L_IR, suggesting no strong correlations between quasar luminosity and star formation luminosity for the most luminous quasars at z > 6.5.