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

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 &gt; 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 &gt; 6. Compared with the 2018 epoch, the 0.5–2 keV flux dropped by a factor of &gt; 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 &gt; 6 will return precious information about the physics of rapid supermassive black hole growth at high redshifts.

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Journal
Astronomy and Astrophysics
Published
2025-02-01
DOI
https://doi.org/10.1051/0004-6361/202453618
Citations
1
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
1.15

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article

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

S. Marchesi, W. N. Brandt, F. E. Bauer, C. Vignali et al.
1 citations
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
1.15
article

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

S. Marchesi, W. N. Brandt, F. E. Bauer, C. Vignali, S. Belladitta, C. Spingola, W. Boschin, D. Paris, Ohad Shemmer, K. Iwasawa, A. Comastri, R. Gilli, G. Chartas, Bin Luo, F. Ricci, F. Cusano, F. Vito, G. Lanzuisi, M. Mignoli
article en
1 citations

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 &gt; 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 &gt; 6. Compared with the 2018 epoch, the 0.5–2 keV flux dropped by a factor of &gt; 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 &gt; 6 will return precious information about the physics of rapid supermassive black hole growth at high redshifts.

Astronomy and AstrophysicsVol. 694
Roma Tre University (IT), Institució Catalana de Recerca i Estudis Avançats (ES), University of North Texas (US), Space Science Institute (US), Pennsylvania State University (US), Pontificia Universidad Católica de Chile (CL), College of Charleston (US), Astronomical Observatory of Rome (IT), Millennium Institute of Astrophysics (CL), Max Planck Institute for Astronomy (DE), Istituto di Radioastronomia di Bologna (IT), Instituto de Astrofísica de Canarias (ES), Telescopio Nazionale Galileo (ES), Osservatorio di Astrofisica e Scienza dello Spazio, Universitat de Barcelona (ES), Clemson University (US), National Institute for Astrophysics (IT), Nanjing University (CN), University of Bologna (IT)
Ohio State University, University of Minnesota, University of Notre Dame, Istituto Nazionale di Astrofisica, Agencia Nacional de Investigación y Desarrollo, Centers for Disease Control and Prevention, Agencia Estatal de Investigación
Openalex Percentile: Top 22%
Astrophysical Phenomena and Observations
1.15
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