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

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.