Changing Look Active Galactic Nuclei

It is known that some active galactic nuclei(AGNs)can transit from Type 1 to Type 2 or vice versa.There are two explanations for the so-called changing-look(CL)AGNs:one is the dramatic change of the obscuration along the line of sight,and the other is the variation of accretion rate.We detected large amplitude variations in the mid-infrared(MIR)luminosity during the transitions in 10 changing-look AGNs us-ing Wide-field Infrared Survey Explorer(WISE)and newly released Near-Earth Object WISE Reactivation(NEOWISE)data.The MIR light curves of 10 objects echo the op-tical variability with a time lag expected for dust reprocessing.And 10 objects featured a MIR color change during their type transition.We find that both the large variability amplitude and the short variability timescale in MIR luminosity are inconsistent with the scenario of varying obscuration,preferring instead to support the scheme of dra-matic change of the accretion rate.Also,we suggest that four CL AGNs might have a second type transition,and their transition was confirmed lately.Then we present the MIR reverberation mapping for four Gaia sources named as Gaial6aax,Gaial6aka,Gaial6ajq,and Gaial5aai.Gaial6aka and Gaial5aai are newly identified Type 1 AGNs by us.Employing the interpolated cross-correlation analysis on the Gaia optical and WISE MIR light curves,we measure the rest-frame lags between the MIR W1/W2 bands and Gaia-G band light curves.We find that the MIR mapping dust radii are?3 times smaller than the silicate sublimation radius,but consistent with that for pure graphite grains.Our results are also consistent with the previous K-band reverberation studies and follows the dust lag-optical luminosity correlation of ???L0.5.CL AGNs can yield considerable insight into accretion physics as well as the co-evolution of black holes and their host galaxies.A large sample of CL AGNs is essential to achieve these goals.We propose an efficient method to select CL candidates from spectroscopic quasar catalogs using the MIR variability information derived from ALL-WISE/NEOWISE data releases.Our primary selection criteria include both a large am-plitude flux variation and a transition of MIR color from an AGN to a normal galaxy.A pilot spectroscopic follow-up of seven candidates among about 300 candidates selected from Sloan Digital Sky Survey low-redshift(z<0.5)AGN sample results in three new turn-off CL AGNs and three ambiguous objects for suffering low spectral quality.Meanwhile,this method can also be used to select turn-on CL candidates.We expect to obtain hundreds of CL AGNs once full spectroscopic follow-up of the sample has been carried out.In addition,we have finished the equipment installation and the test of Double Photometer on 1.2 meter telescope.The software for observation scheduling and the pipeline for data reduction have been developed to achieve efficient observation and convenient raw data processing.We will perform follow-up observations on large sam-ples of AGNs in the future to test AGNs' R-L relation.