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

Context. X-ray emission from quasars (QSOs) has been used to assess supermassive black hole accretion properties up to z 48 6. However, at z > 6 only 4815 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 {MBH}/{M_&09}=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 LUV - LX relation, which is traced by the \u3b1ox 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 \u3b1ox values and bolometric corrections (Kbol = Lbol/LX). We compared the resulting \u3b1ox and Kbol distributions with the results found for QSO samples at lower redshift, and ran several statistical tests to check for a possible evolution of the LUV - LX 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 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 \u3b1ox 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 (\u27e8\u393\u27e9 = 2.20-0.34+0.39 and \u27e8\u393\u27e9 = 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 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

Chandra and Magellan /FIRE follow-up observations of PSO167-13: An X-ray weak QSO at z = 6.515

Context. The discovery of hundreds of quasi-stellar objects (QSOs) in the first gigayear of the Universe powered by already grown supermassive black holes (SMBHs) challenges our knowledge of SMBH formation. In particular, investigations of z  >  6 QSOs that present notable properties can provide unique information on the physics of fast SMBH growth in the early Universe. Aims. We present the results of follow-up observations of the z  =  6.515 radio-quiet QSO PSO167-13, which is interacting with a close companion galaxy. The PSO167-13 system has recently been proposed to host the first heavily obscured X-ray source at high redshift. The goals of these new observations are to confirm the existence of the X-ray source and to investigate the rest-frame UV properties of the QSO. Methods. We observed the PSO167-13 system with Chandra/ACIS-S (177 ks) and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE. Results. No significant X-ray emission is detected from the PSO167-13 system, suggesting that the obscured X-ray source previously tentatively detected was either due to a strong background fluctuation or is highly variable. The upper limit (90% confidence level) on the X-ray emission of PSO167-13 (L2-10  keV  <  8.3 × 1043  erg  s-1) is the lowest available for a z  >  6 QSO. The ratio between the X-ray and UV luminosity of αox  <  -1.95 makes PSO167-13 a strong outlier from the αox - LUV and LX - Lbol relations. In particular, its X-ray emission is more than six times weaker than the expectation based on its UV luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by unfavorable sky conditions, but the tentatively detected C IV and Mg II emission lines appear strongly blueshifted. Conclusions. The most plausible explanations for the X-ray weakness of PSO167-13 are intrinsic weakness or small-scale absorption by Compton-thick material. The possible strong blueshift of its emission lines hints at the presence of nuclear winds, which could be related to its X-ray weakness

Discovery of the first heavily obscured QSO candidate at z > 6 in a close galaxy pair

While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting z > 6 quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first gigayear of the Universe from an observational point of view. We report the Chandra discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close ( 485 kpc) galaxy pair at z = 6.515. One of the members is an optically classified type-1 QSO, PSO167-13. The companion galaxy was first detected as a [C II] emitter by Atacama large millimeter array (ALMA). An X-ray source is significantly (P = 0.9996) detected by Chandra in the 2-5 keV band, with < 1.14 net counts in the 0.5-2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of NH > 2 7 1024 cm-2 and NH > 6 7 1023 cm-2 at 68% and 90% confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at z > 6