Research On A Sample Of AGNs With Strong Flux Variation In Soft X-ray

AGN is considered as the most luminous astronomical object in the Universe. The energetic radiation raises from the gas accretion disk surrounding the central super mas-sive black hole. The broad emission lines come from the broad line region close to the centre black hole. Outwards, in a very large scale there is a narrow line region, where emit narrow lines. There is a torus structure outside the accretion disk and broad line region. Because of the angle of the sight view, it can sometimes obstruct the broad line region and lead the spectra of AGN to appear quite differently. The X-ray emission region is produced close to central super massive black hole. It is believed that the X-ray variability is closely related to the state of the accretion disk and centre black hole. Therefore, research on the X-ray variability will provide us an efficient tool to study the physical picture around the centre SMBH. It can also help us to test some of the related theoretical models. This dissertation include our work on an AGN sample with great flux variation in soft X-ray band.In our work, we focus on sources with strong flux variation in soft X-ray band. By matching ROSAT All-Sky Survey Bright Source Catalogue(1RXS) and XMM-Newton Serendipitous Source Catalogue(2XMMi-DR3), we get a sample including13sources, which have flux variation by a factor of more than10. We consult the literature of those sources which are already studied in this sample and analyze the spectra of those sources which have no previous studies. Then we combine all the information and try to give a reasonable explain of each source’s strong variation.In this paper, considering that the sensitivity of ROSAT is much lower than XMM, we only select those AGN which were bright during ROSAT observations but much fainter during the observations of XMM. This paper is organized as follows:in Chapter1, we present a brief introduction to AGN, including the observational characteristics and classification. We also introduce previous studies on AGN variation, the observed behavior, related physical models, and particularly studies on AGNs with extreme X-ray variation. In Chapter2and3, we present our work, including the two X-ray catalog we adopted to select a sample of AGNs with strong long term soft X-ray variation. The sample is listed, and we discuss their properties through X-ray spectral fitting/analyses and collecting literature studies. We divide the sample into three categories, according to the mechanisms which are responsible for the strong X-ray variation:changes in absorption along the line of sight, light bending effect, and intrinsic variation. In the last Chapter, all the conclusions achieved before are summarized.