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

TL;DR

This study systematically analyzes low-redshift SDSS Type-2 AGN with optical variabilities from Catalina Sky Survey, revealing misclassified Type-1.8/1.9 AGN, true Type-2 AGN, and possible tidal disruption events, advancing understanding of AGN classifications.

Contribution

It provides a large sample analysis of optically selected Type-2 AGN with long-term variability, identifying misclassifications and potential tidal disruption events, which was not previously comprehensively studied.

Findings

156 AGN show significant optical variability indicating misclassification.

31 AGN have broad Hα lines, reclassifying them as Type-1.8/1.9.

14 AGN have multi-epoch spectra with no change in broad lines.

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.