Changing-look AGNs：Discoveries and Challenges

主讲人 (Speaker): 

吴学兵

主讲人单位 (Speaker's Institute): 

北京大学/科维理天文与天体物理研究所

邀请人 (Invited by): 

林伟鹏

时间 (Time): 

星期三, 2019/04/24 - 15:00 to 16:00

地点 (Location): 

珠海校区海滨红楼17栋107 (Rm 107, Red House 17)

摘要 (Abstract): 

       变脸的活动星系核（AGN）是指其紫外/光学光谱中较宽的巴尔默发射线在几年内的短时间里消失，甚至几年后又再出现的特殊天体。它们的发现挑战了我们对AGN统一模型的理解。近年来我们发现了红移在0.08到0.60之间的21个新的变脸AGN样本，把目前已知的此类天体数量增加了一倍。这些新的变脸AGN是通过以下几种方法发现的:(1)SDSS巡天中的重复光谱，(2)LAMOST和SDSS巡天中的重复光谱，(3)光变和新的光谱观测。当变脸AGN开启时，光学和中红外的连续谱辐射变得更亮。在超过3-sigma置信水平的10个变脸AGN中，光学和中红外波段的光变与遮挡变化的模型不一致。我们证实了其光变在光学波段上呈现出更亮更蓝的趋势。然而，其中红外WISE颜色W1-W2在W1波段呈现出变亮而变红，这可能是由于当AGN活动性增强时，W2波段的热尘埃辐射的贡献更强所致。虽然变脸AGN类型转变的物理机制尚不清楚，但观测显示更倾向于与超大质量黑洞周围吸积盘内部吸积率的迅速变化有关。

       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.

主讲人简介 (Speaker's CV): 

Xue-Bing Wu is currently a Professor and the Chair of Department of Astronomy of Peking University, as well as the Associate Director of Kavli Institute for Astronomy & Astrophysics. He got the B.Sc. in 1986 and M.Sc.in 1989 at Central China Normal Univ., and Ph.D. in 1996 at Beijing Astronomical Observatory (now NAOC) of Chinese Academy of Sciences. Then he became a Postdoctoral Researcher at Institute of Theoretical Physics of Chinese Academy of Sciences and a Research Associate at Beijing Astronomical Observatory. Since 2000, he has been working at Peking University as an Associate Professor first and a full Professor later.His research fields include quasar and active galaxies, black hole astrophysics and extragalactic surveys.