摘要：Context.X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up toz ≈ 6. However, atz > 6 only ≈15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of the QSO population in the first Gyr of the Universe.Aims.We present newChandraobservations of a sample of 10z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy$ \left(\log\frac{M_{\mathrm{BH}}}{M_\odot}=8.5{-}9.6\right) $. Adding archival X-ray data for an additional 15z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on theLUV − LXrelation, which is traced by theαoxparameter, and the shape of their X-ray spectra.Methods.We performed photometric analyses to derive estimates of the X-ray luminosities of ourz > 6 QSOs, and thus theirαoxvalues and bolometric corrections (Kbol = Lbol/LX). We compared the resultingαoxandKboldistributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of theLUV − LXrelation. Finally, we performed a basic X-ray spectral analysis of the brightestz > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index.Results.We detect seven of the newChandratargets in at least one standard energy band, while two more are detected discarding energiesE > 5 keV, where background dominates. We confirm a lack of significant evolution ofαoxwith redshift, which extends the results from previous works up toz > 6 with a statistically significant QSO sample. Furthermore, we confirm the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (⟨Γ⟩ = 2.20−0.34+0.39and ⟨Γ⟩ = 2.13−0.13+0.13for sources with < 30 and > 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs atz = 1 − 6.Conclusions.All of these results indicate a lack of substantial evolution of the inner accretion-disk and hot-corona structure in QSOs from low redshift toz > 6. Our data hint at generally high Eddington ratios atz > 6.
关键词：enmethods: data analysisgalaxies: activegalaxies: nucleiX-rays: galaxiesgalaxies: high-redshiftquasars: general
