Intervening nuclear obscuration changing the X-ray look of the $z\approx6$ QSO CFHQS J164121+375520
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arXiv
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2025
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| author | Vito, Fabio Brandt, William Nielsen Comastri, Andrea Gilli, Roberto Bauer, Franz Belladitta, Silvia Chartas, George Iwasawa, Kazushi Lanzuisi, Giorgio Luo, Bin Marchesi, Stefano Mignoli, Marco Ricci, Federica Shemmer, Ohad Spingola, Cristiana Vignali, Cristian Boschin, Walter Cusano, Felice Paris, Diego |
| author_facet | Vito, Fabio Brandt, William Nielsen Comastri, Andrea Gilli, Roberto Bauer, Franz Belladitta, Silvia Chartas, George Iwasawa, Kazushi Lanzuisi, Giorgio Luo, Bin Marchesi, Stefano Mignoli, Marco Ricci, Federica Shemmer, Ohad Spingola, Cristiana Vignali, Cristian Boschin, Walter Cusano, Felice Paris, Diego |
| contents | X-ray observations of the optically selected $z=6.025$ QSO CFHQS J164121+375520 (hereafter J1641) revealed that its flux dropped by a factor $\gtrsim7$ from 2018, when it was a bright and soft X-ray source, to 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 $z>6$. Comparing with the 2018 epoch, the 0.5-2 keV flux dropped dramatically by a factor $>20$. We ascribe this behaviour 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 that increased their thickness. Another possibility is that we have witnessed an occultation event due to dust-free clouds located at sub-pc/pc scales, similar to those recently invoked to explain the remarkable X-ray weakness of AGN discovered by JWST. These interpretations are also consistent with the lack of strong variations of the QSO rest-frame UV lightcurve over the same period. Future monitoring of J1641 and the possible discovery of other X-ray changing look QSOs at $z>6$ will provide us with precious information about the physics of rapid supermassive black-hole growth at high redshift. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2501_12449 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Intervening nuclear obscuration changing the X-ray look of the $z\approx6$ QSO CFHQS J164121+375520 Vito, Fabio Brandt, William Nielsen Comastri, Andrea Gilli, Roberto Bauer, Franz Belladitta, Silvia Chartas, George Iwasawa, Kazushi Lanzuisi, Giorgio Luo, Bin Marchesi, Stefano Mignoli, Marco Ricci, Federica Shemmer, Ohad Spingola, Cristiana Vignali, Cristian Boschin, Walter Cusano, Felice Paris, Diego Astrophysics of Galaxies High Energy Astrophysical Phenomena X-ray observations of the optically selected $z=6.025$ QSO CFHQS J164121+375520 (hereafter J1641) revealed that its flux dropped by a factor $\gtrsim7$ from 2018, when it was a bright and soft X-ray source, to 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 $z>6$. Comparing with the 2018 epoch, the 0.5-2 keV flux dropped dramatically by a factor $>20$. We ascribe this behaviour 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 that increased their thickness. Another possibility is that we have witnessed an occultation event due to dust-free clouds located at sub-pc/pc scales, similar to those recently invoked to explain the remarkable X-ray weakness of AGN discovered by JWST. These interpretations are also consistent with the lack of strong variations of the QSO rest-frame UV lightcurve over the same period. Future monitoring of J1641 and the possible discovery of other X-ray changing look QSOs at $z>6$ will provide us with precious information about the physics of rapid supermassive black-hole growth at high redshift. |
| title | Intervening nuclear obscuration changing the X-ray look of the $z\approx6$ QSO CFHQS J164121+375520 |
| topic | Astrophysics of Galaxies High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2501.12449 |