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

F. Vito, W. N. Brandt, F. E. Bauer, F. Calura, R. Gilli, B. Luo, O. Shemmer, C. Vignali, G. Zamorani, M. Brusa, F. Civano, A. Comastri (+1 others)
2019 Astronomy and Astrophysics  
X-ray emission from QSOs has been used to assess SMBH 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 QSOs in the first Gyr of the Universe. We present new Chandra observations of 10 z>6 QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy. Adding archival X-ray data for an additional 15 z>6 QSOs, we investigate the X-ray
more » ... ties of the QSO population in the first Gyr of the Universe, focusing in particular on the L_UV-L_X relation, which is traced by the α_ox parameter, and the shape of their X-ray spectra. We performed photometric analyses to derive estimates of the X-ray luminosities, and thus the α_ox values and bolometric corrections (K_bol=L_bol/L_X). We compared the resulting α_ox and K_bol distributions with the results found for QSO samples at lower redshift. 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. We confirm a lack of significant evolution of α_ox with redshift, extending the results from previous works up to z>6, and 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^+0.39 and Γ=2.13_-0.13^+0.13 for sources with <30 and >30 net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at z=1-6. All these results point toward a lack of substantial evolution of the inner accretion-disk/hot-corona structure in QSOs from low redshift to z>6. Our data hint at generally high Eddington ratios at z>6.
doi:10.1051/0004-6361/201936217 fatcat:o6nkx3a57nhd5f3lfcgqmbjyse