Mon. Not. R. Astron. Soc., 498, 4033-4050 (2020/November-1)
The frequency of extreme X-ray variability for radio-quiet quasars.
TIMLIN J.D., BRANDT W.N., ZHU S., LIU H., LUO B. and NI Q.
Abstract (from CDS):
We analyse 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (mi <= 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z ≃ 0.1-4 and probes X-ray variability on time-scales of up to ≃12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analysed as a function of time-scale and luminosity. The tail-heavy distributions of variability amplitudes at all time-scales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different time-scales and luminosities. Extreme X-ray variations occur more frequently at long time-scales (Δt >= 300 d) than at shorter time-scales and in low-luminosity quasars compared to high-luminosity quasars over a similar time-scale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of < 2.4 per cent of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
galaxies: active - quasars: general - X-rays: galaxies - X-rays: general
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<Available at CDS (J/MNRAS/498/4033): appena.dat>
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