A High Incidence of Mid-infrared Variability in Local Ultraluminous Infrared Galaxies

We explore mid-infrared (MIR) variability in local ultraluminous infrared galaxies (ULIRGs; infrared luminsoity L IR>10 12 L) utilizing the about 11 years of photometry from the NEOWISE multi-epoch catalog of Wide-field Infrared Survey Explorer (WISE). We identify 30 variable ULIRGs with statistically significant MIR variability. The variability is observed on timescales of a few years, implying that the MIR-emitting regions are compact (1 pc). The difference between maximum and minimum W2 (4.6 μ m) band luminosity (ΔL W2) of the 30 variable ULIRGs range from ΔL W2 = 7 times10 42 to 5 times 10 44 erg s-1. The ΔL W2 of 25 variable ULIRGs out of 30 are greater than ΔL W2 = 1 times10 43 erg s-1, surpassing the MIR luminosity range observed in known supernovae (SNe; L 3.6 μm and L 4.5 μm < 10 42.3 erg s-1). Therefore, the MIR variabilities in these 25 ULIRGs are most likely driven by tidal disruption events (TDEs) or intrinsic changes in their active galactic nuclei (AGN) torus emission. Our sample includes hard X-ray detected AGNs (e.g., UGC 05101) and previously reported TDE candidates (IRAS F01004-2237, IRAS 05189-2524). All 25 also exhibit at least one AGN signature(s) besides the MIR variability, suggesting that even if the MIR variability originates from TDEs, the black holes responsible are likely AGNs. Our results suggest that MIR variability is an effective tool for detecting buried AGNs and highlights the intense nuclear activity in ULIRGs.