<i>Chandra</i> and <i>Magellan</i>/FIRE follow-up observations of PSO167–13: An X-ray weak QSO at <i>z</i> = 6.515
Fabio Vito, W. N. Brandt, F. Ricci, Enrico Congiu, Thomas Connor, Eduardo Bañados, F. E. Bauer, R. Gilli, Bin Luo, Chiara Mazzucchelli, M. Mignoli, Ohad Shemmer, C. Vignali, F. Calura, A. Comastri, Roberto Decarli, S. Gallerani, Riccardo Nanni, M. Brusa, N. Cappelluti, F. Civano, G. Zamorani
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 ( L 2−10 keV < 8.3 × 10 43 erg s −1 ) is the lowest available for a z > 6 QSO. The ratio between the X-ray and UV luminosity of α ox < −1.95 makes PSO167–13 a strong outlier from the α ox − L UV and L X − L bol 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.