Vito, F.F.VitoBrandt, W. N.W. N.BrandtRicci, F.F.RicciCongiu, E.E.CongiuConnor, T.T.ConnorBañados, E.E.BañadosBauer, F. E.F. E.BauerGILLI, RobertoRobertoGILLILuo, B.B.LuoMazzucchelli, C.C.MazzucchelliMIGNOLI, MarcoMarcoMIGNOLIShemmer, O.O.ShemmerVignali, C.C.VignaliCALURA, FrancescoFrancescoCALURACOMASTRI, AndreaAndreaCOMASTRIDECARLI, ROBERTOROBERTODECARLIGallerani, S.S.GalleraniNanni, R.R.NanniBrusa, M.M.BrusaCappelluti, N.N.CappellutiCivano, F.F.CivanoZamorani, G.G.Zamorani2022-03-182022-03-1820210004-6361http://hdl.handle.net/20.500.12386/31706Context. 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.STAMPAenArticle10.1051/0004-6361/2021403992-s2.0-85107079327https://www.aanda.org/articles/aa/abs/2021/05/aa40399-21/aa40399-21.htmlhttps://api.elsevier.com/content/abstract/scopus_id/85107079327FIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation