题目：Changing-look Quasars: Discoveries and Challenges

报告人：吴学兵 教授 (杰青), 北京大学

时间地点：周四, 2018.11.29下午2:30-4:30, 物理楼552

Abstract：

The rare case of changing-look (CL) AGNs, with the appearance or disappearance of broad Balmer 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 doubles 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. The estimated upper limits of transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on. Variations of more than 0.2mag in the mid-infrared WISE/W1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than 3-sigma confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W 1 - W 2 become redder when the objects become brighter in the W 1 band, possibly due to a stronger hot dust contribution in the W2 band when the AGN activity becomes stronger. The physical mechanism of type transition in CL AGNs is important for understanding the evolution of AGNs.

简介：Xue-Bing Wu is an astrophysicist working on quasars, black holes and accretion disks. He is currently the chair of department of astronomy of Peking University and the associate director of Kavli Institute for Astronomy and Astrophysics. He got the doctorate degree in astrophysics from Chinese Academy of Sciences in 1996. After taking several postdoc and visiting positions in Chinese Academy of Sciences, University of Alabama in USA and Max-Planck Institute for Astrophysics in Germany, he joined Peking University in 2000 and became a full professor in 2001. He was awarded the NSFC outstanding youth grant in 2005, the Huang Runqian prize for fundamental astrophysics research by Chinese Astronomical Society in 2015, and the first-class prize in natural sciences of Chinese ministry of education in 2017. In 2015, the discovery of his team on the most luminous quasar with the most massive black hole in the early Universe was selected as one of the top 10 advances in sciences in China by Chinese ministry of science and technology.