Abstract
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 properties 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 $\alpha_ox$ parameter, and
the shape of their X-ray spectra. We performed photometric analyses to derive
estimates of the X-ray luminosities, and thus the $\alpha_ox$ values and
bolometric corrections ($K_bol=L_bol/L_X$). We compared the resulting
$\alpha_ox$ 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 $\alpha_ox$ 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
($\Gamma=2.20_-0.34^+0.39$ and $\Gamma=2.13_-0.13^+0.13$ 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 $z>6$.
Description
The X-ray properties of $z>6$ quasars: no evident evolution of accretion physics in the first Gyr of the Universe
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