A remarkably X-ray and UV variable QSO at cosmic dawn
Abstract
Explaining the existence of supermassive black holes (SMBHs) at z≳6 is a persistent challenge to modern astrophysics. Multi-wavelength observations of z≳6 quasi-stellar objects (QSOs) reveal that, on average, their accretion physics is similar to that of their counterparts at later cosmic times. However, QSOs showing properties that deviate from the general behavior can provide us with useful insights into the physical processes responsible for the rapid growth of SMBHs in the early universe. I will present multi-epoch observations in the X-ray (Chandra and XMM-Newton) and rest-frame UV (LBT, PanSTARRS, and CFHT) bands of a luminous (logL_{bol}=1013 L_⊙) z≈6 QSO, J1641+3755. Its X-ray flux varied by a factor ≳7 from 2018 to 2021, corresponding to ≈115 rest-frame days. Such a large variation in a short timescale for a luminous object is in conflict with our understanding of the general QSO population. In fact, the typical variability amplitude of QSOs exceeds rarely a factor of ≈2, correlates with luminosity, and anti-correlates with the timescale. This finding may be related with the physics of fast accretion in z>6 QSOs required to form >10^8 M_⊙ BHs in <1 Gyr. In fact, I will show that z>6 QSOs present enhanced X-ray variability with respect to systems at later cosmic times. Surprisingly, comparing new LBT photometry with archival data, we found that J1641+3755 shows significant UV brightening by Δmag≈0.4 from 2003 to 2021, in qualitative contradiction with the X-ray fading behaviour. I will discuss possible physical interpretations for the variability properties of J1641, and present on-going multi-wavelength follow-up campaigns on this object.
- Publication:
-
44th COSPAR Scientific Assembly. Held 16-24 July
- Pub Date:
- July 2022
- Bibcode:
- 2022cosp...44.2302V