Discovery of a Mid-infrared Echo from the TDE Candidate in the Nucleus of ULIRG F01004-2237

We present the mid-infrared (MIR) light curves (LCs) of a tidal disruption event candidate in the center of a nearby ultraluminous infrared galaxy F01004-2237 using archival WISE and NEOWISE data from 2010 to 2016. At the peak of the optical flare, F01004-2237 was IR quiescent. About three years later, its MIR fluxes have shown a steady increase, rising by 1.34 and 1.04 mag in 3.4 and 4.6 μm up to the end of 2016. The host-subtracted MIR peak luminosity is 2-3 ×10⁴⁴ erg s^-1. We interpret the MIR LCs as an infrared echo, i.e., dust reprocessed emission of the optical flare. Fitting the MIR LCs using our dust model, we infer a dust torus of the size of a few parsecs at some inclined angle. The derived dust temperatures range from 590-850 K, and the warm dust mass is ∼7 M _o. Such a large mass implies that the dust cannot be newly formed. We also derive the UV luminosity of 4-11 ×10⁴⁴ erg s^-1. The inferred total IR energy is 1-2 ×10⁵² erg, suggesting a large dust covering factor. Finally, our dust model suggests that the long tail of the optical flare could be due to dust scattering.