The X-ray properties of z > 6 quasars: no evident evolution of accretion physics in the first Gyr of the Universe

Abstract

Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z ≈ 6. However, at z > 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. <BR /> Aims: We present new Chandra observations of a sample of 10 z > 6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy (log {M<SUB>BH</SUB>}/{M_☉}=8.5-9.6) . Adding archival X-ray data for an additional 15 z > 6 QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe. In particular, we focus on the L<SUB>UV</SUB> - L<SUB>X</SUB> relation, which is traced by the α<SUB>ox</SUB> parameter, and the shape of their X-ray spectra. <BR /> Methods: We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their α<SUB>ox</SUB> values and bolometric corrections (K<SUB>bol</SUB> = L<SUB>bol</SUB>/L<SUB>X</SUB>). We compared the resulting α<SUB>ox</SUB> and K<SUB>bol</SUB> distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the L<SUB>UV</SUB> - L<SUB>X</SUB> relation. Finally, we performed a basic X-ray spectral analysis of the brightest z > 6 QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index. <BR /> Results: We detect seven of the new Chandra targets in at least one standard energy band, while two more are detected discarding energies E > 5 keV, where background dominates. We confirm a lack of significant evolution of α<SUB>ox</SUB> with redshift, which extends the results from previous works up to z > 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<SUB>-0.34</SUB><SUP>+0.39</SUP> and ⟨Γ⟩ = 2.13<SUB>-0.13</SUB><SUP>+0.13</SUP> for sources with < 30 and > 30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z = 1 - 6. <BR /> 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 to z > 6. Our data hint at generally high Eddington ratios at z > 6.