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

Abstract

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.

Quasar clustering at redshift 6

“…Recent deep X-ray observations of some of the above sources did not reveal potentially obscured quasars. To date, there is no strong evidence that these obscured companions of z ∼ 6 quasars are obscured AGN (Connor et al 2020;Vito et al 2021).…”

Deep XMM-Newton Observations of an X-ray Weak, Broad Absorption Line Quasar at $z=6.5$

“…There are some other high-redshift quasars without X-ray detections, which could thus be candidates of X-ray weak quasars. In particular, Vito et al (2021) report an upper limit on X-ray emission from a z = 6.515 quasar and suggest that its X-ray emission is > 6 times weaker than the expectation based on UV luminosity.…”

“…For high-redshift comparisons, we show individual measurements at 5.5 < z < 6 fromNanni et al (2017), 6 < z < 6.5 fromVito et al (2019a), and z > 6.5 fromWang et al (2021). We also include the new upper limit (black square) from a z = 6.515 quasar inVito et al (2021). The low-redshift SDSS quasar sample (grey dots) is generated based on the L 2−10keV fromTimlin et al (2020) and L 2500 Å fromWang et al (2021).…”

Quasar clustering at redshift 6

“…Recent deep X-ray observations of some of the above sources did not reveal potentially obscured quasars. So far, there is no strong evidence of these obscured companions of z ∼ 6 quasars to be obscured AGN (Connor et al 2020;Vito et al 2021).…”