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

¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
e-mail: fvito.astro@gmail.com
² Instituto de Astrofśica and Centro de Astroingenieria, Facultad de Fśica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
³ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
⁴ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁸ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁰ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
¹¹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
¹³ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹⁴ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
¹⁵ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing, Jiangsu 210093, PR China
¹⁶ Collaborative Innovation Center of Modern Astronomy and Space Exploration, Nanjing 210093, PR China
¹⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹⁸ Department of Physics, University of North Texas, Denton, TX 76203, USA
¹⁹ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
²⁰ Physics Department, University of Miami, Coral Gables, FL 33124, USA
²¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden st, Cambridge, MA 02138, USA

Received: 22 January 2021
Accepted: 10 March 2021

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 × 10⁴³ 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.