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

TL;DR

This paper addresses the rarity and physics of repeating changing-look AGNs by building a large, multi-epoch, spectroscopic and MIR-variability dataset. It identifies 25 RCL AGNs among 331 CL AGNs (with 19 new confirmations), using a combination of quantitative BEL-flux variation ($R_{ m s} > 0.3$) and careful visual inspection, aided by MIR light curves. Turn-on transition timescales are derived for 19 RCL AGNs, with no strong correlation found between timescale and black hole mass, challenging simple accretion-rate change scenarios. The expanded RCL AGN sample enhances constraints on CL-cycle durations and provides a valuable benchmark for testing accretion physics in active galaxies.

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.