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

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Abstract

Large-amplitude X-ray variation is a special feature of AGN, reflecting possible extreme change in the central engine or the absorption 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 variation 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.

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Authors and Affiliations

  1. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, 230026, China

    Da Bi, Teng Liu & JunXian Wang

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  1. Da Bi
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  2. Teng Liu
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Correspondence to JunXian Wang.

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Bi, D., Liu, T. & Wang, J. A sample of active galactic nuclei with strong soft X-ray variabilities. Sci. China Phys. Mech. Astron. 58, 1–8 (2015). https://doi.org/10.1007/s11433-014-5607-x

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  • DOI: https://doi.org/10.1007/s11433-014-5607-x

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