标题： 四例因光学变化而显现的活动星系核 显示英文标题

标题： Four Changing-Look Active Galactic Nuclei Found From Optical Variations

摘要： 我们报告了四个变脸（CL）活动星系核（AGN）的发现。由于它们相对较大的光学流量变化和相关的中红外流量变化，我们认为这些源可能是有趣的观测目标。为了识别它们的CL特征，我们使用了至少8年前从斯隆数字巡天（SDSS）存档光谱以及最近从暂现源名称服务器（TNS）和2.4米丽江望远镜（LJT）获得的光谱。我们通过拟合和确定H$_\alpha$和H$_\beta$成分来研究这些源的光谱变化，并验证它们的CL行为。当比较TNS和/或LJT光谱与SDSS光谱时，所有四个源都显示出宽的或更强的宽H$_\alpha$成分和相对较弱的宽H$_\beta$成分的出现。由于其中两个源在测光数据中表现出更亮且更蓝的特性，在TNS和LJT光谱拍摄的时间段内，这一特性可能伴随着宽成分的开启。因此，我们认为这种更亮且更蓝的特性可以作为有效识别先前光谱分类为2型AGN的CL源的标准，例如来自SDSS提供的源。 显示英文摘要

摘要： We report the finding of four changing-look (CL) active galactic nuclei (AGN). We selected these sources due to their potential as interesting targets when considering their relatively-large optical flux variations and related mid-infrared flux variations. To identify their CL feature, we use archival spectra from the Sloan Digital Sky Survey (SDSS) taken at least 8 years ago as well as spectra taken recently from the Transient Name Server (TNS) and with the 2.4-m LiJiang telescope (LJT). We study the sources' spectral changes by fitting and determining the H$_\alpha$ and H$_\beta$ components and verify their CL behavior. When comparing the TNS and/or LJT spectra to the SDSS ones, all four sources showed the appearance of a broad or a stronger broad H$_\alpha$ component and a relatively weak broad H$_\beta$ component. As two of the four sources are established to have a brighter-and-bluer feature in the photometric data, during the time periods in which the TNS and LJT spectra were taken, this feature likely accompanied the turn-on of the broad components. Thus, we suggest that this brighter-and-bluer feature can be used as a criterion for efficiently finding CL sources among previously spectroscopically classified type 2 AGN, such as from among the sources provided by the SDSS.