The Relation between X-ray and Ultraviolet Variability of Quasars

• Published in: arXiv.org
• Main Authors: Sou, Hao, Jun-Xian, Wang, Zhang-Liang, Xie, Wen-Yong, Kang, Zhen-Yi Cai
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 14.04.2022
• Subjects: Active galactic nuclei; Brightening; Deposition; Dimming; Physical properties; Physics - Astrophysics of Galaxies; Physics - High Energy Astrophysical Phenomena; Quasars; Reprocessing; Switches; Time lag; Variability; XMM (spacecraft)
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2203.07119

The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFx-logFuv space. After correcting for the effect of photometric noise, we find 35.6\pm2.1% of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8\pm4.2% of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ~ 2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disk model which incorporates both X-ray and UV/optical variability in AGNs is favored by the observed stochastic relation between X-ray and UV variations.