Systematic Monitoring of Extreme X-ray Variability from Weak-line Quasars

Abstract

We present the results of a multi-cycle Chandra program to systematically monitor the X-ray variability of 10 weak-line quasars (WLQs) that previously had limited multi-epoch X-ray observations. Three new Chandra 2.8 to 8.2 ks observations were obtained for each WLQ with C$\,$IV rest-frame equivalent widths (REWs) $\lesssim 10$ \r{A}, substantially improving the monitoring data quality of WLQs and our ability to characterize their long-term X-ray variability behavior. We observe recurrent extreme X-ray variability in the historically variable WLQ SDSS J1539+3954, with an X-ray flux rise of a factor of $\gtrsim 6$ between 2023 and 2024 ($\gtrsim 21$ relative to 2013). Another previously X-ray weak WLQ in the sample, SDSS J0825+1155, underwent a significant X-ray flux variation by a factor of $\gtrsim 14$ between 2019 and 2023. We find the fraction of WLQs exhibiting evidence of extreme X-ray variability to be $0.20^{+0.17}_{-0.07}$. In the context of the thick disk and outflow (TDO) model, the substantial fraction of WLQs displaying extreme X-ray variability may suggest that the variability is more likely driven by the intrinsic motion of the TDO wind rather than changes in the height of the TDO disk. We performed a statistical comparison between the distribution of variability amplitudes of WLQs and general radio-quiet quasars. We find that these underlying distributions are statistically different, with WLQs having $\approx 6.8$ times higher odds of exhibiting an extreme X-ray variability event than the general radio-quiet quasar population.