Intervening nuclear obscuration changing the X-ray look of the $z\approx6$ QSO CFHQS J164121+375520

Fabio Vito, William Nielsen Brandt, Andrea Comastri, Roberto Gilli, Franz Bauer, Silvia Belladitta, George Chartas, Kazushi Iwasawa, Giorgio Lanzuisi, Bin Luo, Stefano Marchesi, Marco Mignoli, Federica Ricci, Ohad Shemmer, Cristiana Spingola, Cristian Vignali, Walter Boschin, Felice Cusano, Diego Paris

First listed 2025-01-21 | Last updated 2025-02-03

Abstract

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.

Short digest

The authors re-monitor the z=6.025 QSO CFHQS J164121+375520 (J1641) with Chandra and rest‑UV imaging in 2022–2024 after a dramatic X-ray dimming seen between 2018 and 2021. J1641 is now detected only in the hard 2–7 keV band, while its soft 0.5–2 keV flux has faded by >20× relative to 2018, establishing it as an X‑ray changing‑look quasar at z>6. They attribute the transition to intervening, ongoing Compton‑thick obscuration along our line of sight—potentially a thickened inner disk/failed wind or an occultation by dust‑free sub‑pc clouds—with the near‑constant rest‑UV light curve supporting obscuration over intrinsic fading. Catching such events in the reionization era offers a new handle on how rapidly growing SMBHs are fueled and shrouded.

Key figures to inspect

• Figure 1 (soft-band panels): Compare 2018 versus the 2022–2024 stack at 0.5–2 keV to see the disappearance of soft photons within the photometric aperture, which drives the >20× soft-band dimming and the changing‑look classification.
• Figure 1 (hard-band panels): Inspect the 2–7 keV images to confirm J1641’s re‑detection only at higher observed energies in 2022–2024, consistent with heavy line‑of‑sight absorption while the source remained bright in 2018.
• Figure 2 (X-ray/UV timeline): Read off the soft- and hard-band flux evolution—soft dropping by >20× while the hard band shows at most modest dimming—and the overplotted i‑band magnitudes, which remain stable, reinforcing an obscuration scenario.