The fall of CSS100217: a tidal disruption-induced low state in an apparently hostless active galactic nucleus

CSS100217 was a nuclear, rapid, and luminous flare in a narrow-line seyfert 1 galaxy, whose initial interpretation as a supernova is now debated between variability of the active galactic nucleus (AGN) and a tidal disruption event (TDE). In this paper, we present and discuss new evidence in favour of a TDE or extreme flaring episode scenario. After the decay of the flare, the galaxy entered a long-term low luminosity state, 0.4 mag lower than the pre-outburst emission in the V band. We attribute this to the creation of a cavity in the accretion disc after the tidal disruption of a star in a retrograde orbit with respect to the accretion disc rotation, making a TDE our favoured interpretation of the flare. We also show how the host galaxy shows a point-like, compact profile, no evidence for an extended component and a relatively low mass, unlike what expected from an AGN host galaxy at z = 0.147. A compact host galaxy may result in an increased TDE rate, strengthening our interpretation of the event.