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 (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 the QSO population in the first Gyr of the Universe. 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 BH 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, 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. Methods. We performed photometric analyses to derive estimates of the X-ray luminosities of our z > 6 QSOs, and thus their 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, and ran several statistical tests to check for a possible evolution of the L UV − L X 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. Results. We detected 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 ox with redshift, extending the results from previous works up to z > 6 with a statistically significant QSO sample, 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 +0.39 −0.34 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. Conclusions. 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.

First constraints on the AGN X-ray luminosity function atz~ 6 from an eROSITA-detected quasar

“…The photon index remains poorly constrained but is consistent with typical X-ray spectral slopes of the radio quiet quasar population. spectral analysis of X-ray-detected z > 6 quasars, Vito et al (2019) derived an average photon index of Γ = 2.20 +0.22 −0.20 , a value consistent with earlier results by Nanni et al (2017) who performed the same exercise for a z > 5.7 quasar sample.…”

“…Reanalysing the Chandra data with a slightly lower photon index (Γ = 1.9), Nanni et al (2017) derived the rest-frame intrinsic luminosity L 2.0−10.0kev = 4.2 +1.0 −1.4 × 10 45 erg s −1 . With respect to the complete sample of X-ray detected z > 5.7 quasars (Nanni et al 2017;Vito et al 2019;Pons et al 2020), this makes SDSSJ08 one of the four most X-ray luminous high-redshift A5, page 4 of 16 J. Wolf et al: eROSITA high-z quasar Table 2. Derived X-ray properties.…”

“…Chandra and XMM-Newton pointed observations of known quasars in the range z = 5.7−7.54 have led to the detection of an X-ray signal for only approximately 30 of these objects (e.g. Brandt et al 2002;Nanni et al 2017;Vito et al 2019;Pons et al 2020). However, such X-ray follow-up samples suffer from the selection biases of the optical selection because of tight colour-magnitude constraints and absorption.…”

What do observations tell us about the highest-redshift supermassive black holes?

“…An increasing number of z ∼ > 6 quasars have now been detected in the X-rays (e.g., Shemmer et al 2006;Bañados et al 2018b;Vito et al 2019;Pons et al 2020; see compilation by Nanni et al 2017). The strength of their X-ray emission follows the expectations from lower-redshift quasars, namely the close relation between the (restframe) UV luminosity and UV-to-X-ray spectral slope (α ox ; e.g., Lusso & Risaliti 2016;Fig.…”

“…Top: the (rest-frame) near-UV spectrum of ULAS J1342+0928, the highest redshift quasar known to date (z = 7.54), compared with a composite spectrum of lower-z SDSS quasars (taken from Bañados et al 2018a). Bottom: the deviation in the optical-to-X-ray spectral slope, αox, compared with the αox − LUV relation calibrated at lower z AGN (e.g., Lusso & Risaliti 2016), vs. redshift (figure taken from Vito et al 2019).…”