Revealing the accretion physics of supermassive black holes at redshift z~7 with chandra and infrared observations

X-ray emission from quasars has been detected up to redshift z = 7.5, although only limited to a few objects at z > 6.5. In this work, we present new Chandra observations of five z > 6.5 quasars. By combining with archival Chandra observations of six additional z > 6.5 quasars, we perform a systematic analysis on the X-ray properties of these earliest accreting supermassive black holes (SMBHs). We measure the black hole masses, bolometric luminosities (Lbol), Eddington ratios (λEdd), emission line properties, and infrared luminosities (LIR) of these quasars using infrared and submillimeter observations. Correlation analysis indicates that the X-ray bolometric correction (the factor that converts from X-ray luminosity to bolometric luminosity) decreases with increasing Lbol, and that the UV/optical-to-X-ray ratio, αox, strongly correlates with L2500 Å, and moderately correlates with λEdd and blueshift of C IV emission lines. These correlations are consistent with those found in lower-z quasars, indicating quasar accretion physics does not evolve with redshift. We also find that LIR does not correlate with L2-10 keV in these luminous distant quasars, suggesting that the ratio of the SMBH growth rate and their host galaxy growth rate in these early luminous quasars are different from those of local galaxies. A joint spectral analysis of the X-ray detected z > 6.5 quasars yields an average X-ray photon index of Γ; = 2.32-0.30+0.31, steeper than that of low-z quasars. By comparing it with the Γ - λEdd relation, we conclude that the steepening of Γ for quasars at z > 6.5 is mainly driven by their higher Eddington ratios.