The rebrightening of a ROSAT-selected tidal disruption event: repeated weak partial disruption flares from a quiescent galaxy?

The ROSAT-selected tidal disruption event (TDE) candidate RX J133157.6-324319.7 (J1331) was detected in 1993 as a bright [0.2-2 keV flux of (1.0 +/- 0.1) x 10(-12) erg s(-1) cm(-2)], ultra-soft (kT = 0.11 +/- 0.03 keV) X-ray flare from a quiescent galaxy (z = 0.051 89). During its fifth all-sky survey (eRASS5) in 2022, Spectrum-Roentgen-Gamma (SRG)/ eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2-2 keV flux of (6.0 +/- 0.7) x 10(-13) erg s(-1) cm(-2), with spectral properties (kT = 0.115 +/- 0.007 keV) consistent with the ROSAT-observed flare similar to 30 yr earlier. In this work, we report on X-ray, ultraviolet, optical, and radio observations of this system. During a pointed XMM observation similar to 17 d after the eRASS5 detection, J1331 was not detected in the 0.2-2 keV band, constraining the 0.2-2 keV flux to have decayed by a factor of ?40 over this period. Given the extremely low probability (similar to 5 x 10(-6)) of observing two independent full TDEs from the same galaxy over a 30 yr period, we consider the variability seen in J1331 to be likely caused by two partial TDEs involving a star on an elliptical orbit around a black hole. J1331-like flares show faster rise and decay time-scales [O(d)] compared to standard TDE candidates, with negligible ongoing accretion at late times post-disruption between outbursts.

Automated summary

The ROSAT-selected tidal disruption event (TDE) candidate RX J133157.6-324319.7 (J1331) was detected as a bright and ultra-soft X-ray flare in 1993 from a quiescent galaxy. During the recent eRASS5, J1331 exhibited repeated flaring with similar spectral properties to the previous flare, suggesting two partial TDEs involving a star on an elliptical orbit around a black hole.