Discovery of Repeating Transitions in 25 Changing-look Active Galactic Nuclei

TL;DR

This study identifies 25 repeating changing-look AGNs using multi-epoch spectroscopy and infrared data, revealing that repeated transitions are relatively common and providing insights into their transition timescales and underlying physics.

Contribution

It expands the sample of known RCL AGNs by 19 new cases and derives transition timescales, offering new constraints on AGN variability mechanisms.

Findings

8% occurrence rate of RCL AGNs among CL AGNs

19 new RCL AGNs identified

No correlation between transition timescale and black hole mass

Abstract

The repeating changing-look active galactic nuclei (RCL AGNs) exhibit multiple appearances and disappearances of broad emission lines (BELs), whose underlying mechanism remains a puzzle. Expanding the sample of RCL AGNs is valuable for constraining the transition timescale and probing the accretion physics driving CL behaviors. This study aims to identify RCL AGNs using the multi-epoch spectroscopic data of confirmed CL AGNs from the Sloan Digital Sky Survey, Large Sky Area Multi-Object Fiber Spectroscopic Telescope, and Dark Energy Spectroscopic Instrument, supplemented with mid-infrared (MIR) light curves. Through selection criteria and visual inspection, we identify 25 RCL AGNs among 331 CL AGNs, corresponding to an occurrence rate of about $8\%$, indicating that repeated transitions are not rare in CL AGNs. Among the 25 RCL AGNs, 19 are newly identified, which significantly expands the known RCL AGN sample. Based on the spectra and densely sampled MIR light curves, we derive reliable turn-on transition timescales for 19 RCL AGNs, and find no significant correlation between the timescale and the black hole mass.