The nuclear transient AT 2017gge: a tidal disruption event in a dusty and gas-rich environment and the awakening of a dormant SMBH

Abstract

We present the results from a dense multi-wavelength (optical/UV, (IR), and X-ray) followup campaign of the nuclear transient AT 2017gge, covering a total of 1698 days from the transient's discovery. The bolometric light-curve, the black body temperature and radius, as well as the broad H and He 𝜆5876 emission lines and their evolution with time, are all consistent with a TDE nature. A soft X-ray flare is detected with a delay of ∼200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He 𝜆4686 and by a number of long-lasting high ionization coronal emission lines. An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He 𝜆10830. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.