Infrared observations reveal the reprocessing envelope in the tidal disruption event AT 2019azh

Context. Tidal disruption events (TDEs) are expected to release much of their energy in the far-ultraviolet (UV), which we do not observe directly. However, infrared (IR) observations can observe re-radiation of the optical/UV emission from dust, and if this dust is observed in the process of sublimation, we can infer the un-observed UV radiated energy. Tidal disruption events have also been predicted to show spectra shallower than a blackbody in the IR, but this has not yet been observed. Aims. We present IR observations of the TDE AT 2019azh that span from −3 d before the peak until > 1750 d after. We evaluate these observations for consistency with dust emission or direct emission from the TDE. Methods. We fitted the IR data with a modified blackbody associated with dust emission. We compared the UV+optical+IR data with simulated spectra produced from general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion. We modelled the data at later times (> 200 d) as an IR echo. Results. The IR data at the maximum light cannot be self-consistently fitted with dust emission. Instead, the data can be better fitted with a reprocessing model, with the IR excess arising due to the absorption opacity being dominated by free-free processes in the dense reprocessing envelope. We infer a large viewing angle of ∼60°, which is consistent with previously reported X-ray observations, and a tidally disrupted star with a mass > 2 M⊙. The IR emission at later times is consistent with cool dust emission. We modelled these data as an IR echo and found that the dust is distant (0.65 pc) and clumpy, with a low covering factor. We show that TDEs can have an IR excess that does not arise from dust and that IR observations at early times can constrain the viewing angle for the TDE in the unified model. Near-IR observations are therefore essential to distinguish between hot dust and a non-thermal IR excess.