The Type 1 and Type 2 AGN dichotomy according to their ZTF optical variability

The scarce optical variability studies in spectrally classified Type 2 active galactic nuclei (AGNs) have led to the discovery of anomalous objects that are incompatible with the simplest unified models (UMs). This paper focuses on the exploration of different variability features that allow to distinguish between obscured, Type 2 AGNs and the variable, unobscured Type 1s. We analyse systematically the Zwicky Transient Facility, 2.5-yr-long light curves of similar to 15000 AGNs from the Sloan Digital Sky Survey Data Release 16, which are generally considered Type 2s due to the absence of strong broad emission lines (BELs). Consistent with the expectations from the UM, the variability features are distributed differently for distinct populations, with spectrally classified weak Type 1s showing one order of magnitude larger variances than the Type 2s. We find that the parameters given by the damped random walk model lead to broader H alpha equivalent width for objects with tau(g) > 16 d and long-term structure function SF infinity, g > 0.07 mag. By limiting the variability features, we find that similar to 11 percent of Type 2 sources show evidence for optical variations. A detailed spectral analysis of the most variable sources (similar to 1 percent of the Type 2 sample) leads to the discovery of misclassified Type 1s with weak BELs and changing-state candidates. This work presents one of the largest systematic investigations of Type 2 AGN optical variability to date, in preparation for future large photometric surveys.

Automated summary

This study explores different variability features to distinguish between obscured Type 2 AGNs and variable Type 1s. By analyzing the Zwicky Transient Facility light curves of 15000 AGNs, it is found that weak Type 1s exhibit larger variances than Type 2s. Approximately 11% of Type 2 sources show evidence for optical variations when limiting the variability features.