A high incidence of mid-infrared variability in local ultraluminous infrared galaxies

Abstract We explore mid-infrared (MIR) variability in local ultraluminous infrared galaxies (ULIRGs; infrared luminosity $L_{\rm IR}\gt 10^{12}\, {L}_\odot$) utilizing the $\sim$11 yr of photometry from the NEOWISE multi-epoch catalog of the Wide-Field Infrared Survey Explorer (WISE). We identify 30 variable ULIRGs with statistically significant MIR variability. The variability is observed on timescales of a few years, implying that the variable MIR-emitting regions are compact ($\lesssim$a few pc). The difference between maximum and minimum W2 (4.6 ${\rm \mu }$m) band luminosity ($\Delta L_\mathit {W2}$) of the 30 variable ULIRGs ranges from $\Delta L_\mathit {W2}$ = $7\times 10^{42}$ to $5\times 10^{44}$ erg s$^{-1}$. The $\Delta L_\mathit {W2}$ of 25 variable ULIRGs out of 30 are greater than $\Delta L_\mathit {W2}$ = $1\times 10^{43}$ erg s$^{-1}$, surpassing the MIR luminosity range observed in known supernovae (SNe; $L_{\rm 3.6\, {\rm \mu m}}$ and $L_{\rm 4.5\, {\rm \mu m}}$ < 10$^{42.3}$ erg s$^{-1}$). Therefore, the MIR variabilities in these 25 ULIRGs are most likely driven by tidal disruption events (TDEs) or active galactic nuclei (AGN) activities. 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 highlight intense nuclear activity in ULIRGs.