Letter to the Editor

Intervening nuclear obscuration changing the X-ray look of the z ≈ 6 quasi-stellar object CFHQS J164121+375520

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
² Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
³ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁴ Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul, 7820436 Santiago, Chile
⁶ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul, 7820436 Santiago, Chile
⁷ Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
⁸ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁹ Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁰ Department of Physics and Astronomy, College of Charleston, Charleston, SC 29424, USA
¹¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹² ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
¹³ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
¹⁴ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing, Jiangsu 210093, PR China
¹⁵ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, via Gobetti 93/2, I-40129 Bologna, Italy
¹⁶ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
¹⁷ Dipartimento di Matematica e Fisica, Universitá Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy
¹⁸ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00078 Monte Porzio Catone, Italy
¹⁹ Department of Physics, University of North Texas, Denton, TX 76203, USA
²⁰ INAF – Istituto di Radioastronomia, via Gobetti 101, I-40129 Bologna, Italy
²¹ Fundación Galileo Galilei – INAF (Telescopio Nazionale Galileo), Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
²² Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, E-38205 La Laguna (Tenerife), Canary Islands, Spain
²³ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, E-38205 La Laguna (Tenerife), Canary Islands, Spain

^⋆ Corresponding author; fabio.vito@inaf.it

Received: 27 December 2024
Accepted: 21 January 2025

Abstract

X-ray observations of the optically selected z = 6.025 quasi-stellar object (QSO) CFHQS J164121+375520 (hereafter J1641) revealed that its flux dropped by a factor of ≳7 between 2018, when it was a bright and soft X-ray source, and 2021. Such a strong variability amplitude has not been observed before among z > 6 QSOs, and the underlying physical mechanism was unclear. We carried out a new X-ray and rest-frame UV monitoring campaign of J1641 over 2022–2024. We detected J1641 with Chandra in the 2–7 keV band, while no significant emission is detected at softer X-ray energies, making J1641 an X-ray changing-look QSO at z > 6. Compared with the 2018 epoch, the 0.5–2 keV flux dropped by a factor of > 20. We ascribe this behavior to intervening, and still ongoing, obscuration by Compton-thick gas intercepting our line of sight between 2018 and 2021. The screening material could be an inner disk or a failed nuclear wind whose thickness increased. Another possibility is that we have witnessed an occultation event due to dust-free clouds located at parsec or subparsec scales, similar to those recently invoked to explain the remarkable X-ray weakness of active galactic nuclei discovered by JWST. These interpretations are also consistent with the lack of strong variations in the QSO rest-frame UV light curve over the same period. Future monitoring of J1641 and the possible discovery of other X-ray changing look QSOs at z > 6 will return precious information about the physics of rapid supermassive black hole growth at high redshifts.