SHAO Astrophysics Colloquium

Title：Changing-look AGNs: Discoveries and Challenges

Speaker: Xue-Bing Wu (Peking University)

Location: Lecture hall, 3rd floor

Time: 3:00pm on Thursday (Sept.19)

Abstract:The discovery of changing-look (CL) AGNs, with the appearance or disappearance of broad emission lines in the UV/optical spectra within a few years, challenges our understanding of the AGN unification model. We present a sample of 21 new CL AGNs at redshifts between 0.08 and 0.60, which significantly increases the number of such objects known to date. These new CL AGNs were discovered by several ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the LAMOST and SDSS, and (3) photometric variability and new spectroscopic observations. We confirm a bluer-when-brighter trend in the optical band. However, the mid-infrared WISE colors become redder when the objects become brighter, possibly due to a stronger hot dust re-radiation when the central AGN activity becomes stronger. The physical mechanism of type transition in CL AGNs is still unknown, but preferred to be related to the accretion rate change in the inner part of accretion disks around supermassive black holes. The analogy of state transitions in CL AGNs and black hole X-ray binaries will be discussed.