|Time:||Thursday, December 20, 2018, 02:00pm|
|Title:||Changing-look Quasars: Discoveries and Challenges|
|Speaker:||Prof. Xuebing Wu (Peking University)|
The discovery 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 continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on. 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 W1 - W2 become redder when the objects become brighter in the W1 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 still unknown, but preferred to be related to the accretion rate change in the inner part of accretion disks around super massive black holes.
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 and joined Peking University in 2000 as a faculty member after taking several postdoc and visiting positions in China, USA and Germany. 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.
Host: Prof. Cheng Li