A sample of active galactic nuclei with strong soft X-ray variabilities

• Published in: Science China. Physics, mechanics & astronomy mechanics & astronomy, 2015-04, Vol.58 (4), p.101-108
• Main Authors: Bi, Da, Liu, Teng, Wang, JunXian
• Format: Article
• Language: English
• Subjects: Absorption; Active galactic nuclei; Astronomy; Classical and Continuum Physics; Construction; Coronas; Fluxes; Line of sight; Observations and Techniques; Physics; Physics and Astronomy; RASS; ROSAT; Seyfert galaxies; Sky surveys (astronomy); Soft x rays; Spectra; X ray spectra; X-ray astronomy; X-rays; XMM (spacecraft); 光线弯曲; 变异; 样本; 活动星系核; 赛弗特星系; 软X射线
• ISSN: 1674-7348; 1869-1927
• DOI: 10.1007/s11433-014-5607-x

Large-amplitude X-ray variation is a special feature of AGN, reflecting possible extreme change in the central engine or the ab- sorption along the line of sight. Till now there are only a few relevant studies on individual sources or rather small samples. In this work we aim to perform a systematic study of AGNs with strong soft X-ray variations at timescale of 〉 10 years. To build the sample, we compare the soft X-ray fluxes of AGNs measured in ROSAT All-Sky Survey （RASS） during 1990-1991 with those measured by XMM-Newton during 2000-2009. This investigation bings forth to a sample of 13 AGNs, which appeared bright in RASS era, and went into low states （flux dropped by a factor of 〉 10） when they were caught by XMM-Newton. Most of the 13 sources are type I Seyfert galaxies. 5 of them are noticed to have strong X-ray vai＇iation for the first time. We study the nature of their variations through performing XMM-Newton spectral analyses and by collecting reports from the existing literature. We find the sample could be divided into three categories according to the possible causes of the strong X-ray variations. The variations in MRK 0478 and 1H 0419-557 are consistent with strong light-bending effect, i.e., the observed X-ray flux drops significantly as the X-ray emitting corona gets much closer to the central black hole. The variations in ESO 140-G043 and NGC 7158 are caused by absorption changes along the line of sight. For one special case MRK 0335, the variation can be explained by either light-bending or absorption variation. In the rest 8 sources （- 60%）, the strong soft X-ray variations are likely to exist due to intrinsic changes in the activities of the corona, although in some of them without high quality X-ray spectra we are unable to rule out alternative models. This sample provides good targets for future monitoring campaigns with more extensive studies.