2022ApJ...924L..35L

The X-ray source 3XMM J150052.0+015452 was discovered as a spectacular tidal disruption event candidate during a prolonged (>11 yr) outburst. It exhibited unique quasi-soft X-ray spectra of characteristic temperature kT ∼ 0.3 keV for several years at the peak, but in a recent Chandra observation (10 yr into the outburst) a super-soft X-ray spectrum of kT ∼ 0.15 keV was detected. Such dramatic spectral softening could signal the transition from the super-Eddington to thermal state or the temporary presence of a warm absorber. Here we report on our study of four new XMM-Newton follow-up observations of the source. We found that they all showed super-soft spectra, suggesting that the source had remained super-soft for >5 yr. Then its spectral change is best explained as due to the super-Eddington to thermal spectral state transition. The fits to the thermal-state spectra suggested a smaller absorption toward the source than that obtained in Lin et al. This led us to update the modeling of the event as due to the disruption of a 0.75 M_☉ star by a massive black hole of a few x 10⁵ M_☉. We also obtained two HST images in the F606W and F814W filters and found that the dwarf star-forming host galaxy can be resolved into a dominant disk and a smaller bulge. No central point source was clearly seen in either filter, ruling out strong optical emission associated with the X-ray activity.