Systematic research of low redshift optically selected SDSS Type-2 AGN but with apparent long-term optical variabilities from Catalina Sky Survey, I: data sample and basic results

Abstract

The main objective of the paper, the first paper in a dedicated series, is to report basic results on systematic research of low-redshift optically selected SDSS Type-2 AGN but with apparent optical variabilities. For all the pipeline classified Type-2 AGN in SDSS DR16 with $z<0.3$ and $SN>10$, long-term optical V-band light curves are collected from Catalina Sky Survey. Through all light curves described by Damped Random Walk process with process parameters of $\sigma/(mag/days^{0.5})$ and $\tau/days$, 156 Type-2 AGN have apparent variabilities with process parameters at least three times larger than corresponding uncertainties and with $\ln(\sigma/(mag/days^{0.5}))>-4$, indicating central AGN activity regions directly in line-of-sight, leading the 156 Type-2 AGN as mis-classified Type-2 AGN. Furthermore, based on spectroscopic emission features around H$\alpha$, 31 out of the 156 AGN have broad H$\alpha$, indicating the 31 Type-2 AGN are actually Type-1.8/1.9 AGN. Meanwhile, 14 out of the 156 AGN have multi-epoch SDSS spectra. After checking multi-epoch spectra of the 14 objects, no clues for appearance/disappearance of broad lines indicate true Type-2 AGN rather than changing-look AGN are preferred in the collected Type-2 AGN with long-term variabilities. Moreover, a small sample of Type-2 AGN have long-term variabilities with features roughly described by theoretical TDEs expected $t^{-5/3}$, indicating probable central TDEs as further and strong evidence to support true Type-2 AGN.