Rapid onset of a Comptonisation zone in the repeating tidal disruption event XMMSL2 J140446.9-251135

We report here on observations of a tidal disruption event (TDE) discovered in an XMM-Newton slew, in a quiescent galaxy at z=0.043. X-ray monitoring covered the epoch when the accretion disc transitioned from a thermal state, with kT∼80 eV, to a harder state dominated by a warm, optically thick corona. The bulk of the coronal formation took place within 7 days and was coincident with a temporary drop in the emitted radiation by a factor 4. After a plateau phase of ∼ 100 days, the X-ray flux of decayed by a factor 500 within 230 days. We estimate the black hole mass in the galaxy to be M_ ̊m BH and the peak X-ray luminosity L_ ̊m X ∼6 łumUnitsns. The optical/UV light curve is flat over the timescale of the observations with L_ ̊m opt ∼ 2 łumUnitsns. We find that TDEs with coronae are more often found in an X-ray sample than in an optically selected sample. Late-time monitoring of the optical sample is needed to test whether this is an intrinsic property of TDEs or is due to a selection effect. From the fast decay of the X-ray emission we consider that the event was likely due to the partial stripping of an evolved star rather than a full stellar disruption, an idea supported by the detection of two further re-brightening episodes, two and four years after the first event, in the SRG/eROSITA all-sky survey.