First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z < 0.6

We present the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. These 13 events were selected among X-ray transients detected in the 0 degrees < l < 180 degrees hemisphere by eROSITA during its second sky survey (2020 June 10 to December 14) and confirmed by optical follow-up observations. The most distant event occurred at z = 0.581. One TDE continued to brighten at least 6 months. The X-ray spectra are consistent with nearly critical accretion on to black holes of a few x 10(3) to 10(8) M-circle dot, although supercritical accretion is possibly taking place. In two TDEs, a spectral hardening is observed 6 months after the discovery. Four TDEs showed an optical brightening apart from the X-ray outburst. The other nine TDEs demonstrate no optical activity. All 13 TDEs are optically faint, with L-g/L-X < 0.3 (L-g and L-X being the g band and 0.2-6 keV luminosity, respectively). We have constructed a TDE X-ray luminosity function, which can be fit by a power law with a slope of -0.6 +/- 0.2, similar to the trend observed for optically selected TDEs. The total rate is estimated at (1.1 +/- 0.5) x 10(-5) TDEs per galaxy per year, an order of magnitude lower than inferred from optical studies. This suggests that X-ray bright events constitute a minority of TDEs, consistent with models predicting that X-rays can only be observed from directions close to the axis of a thick accretion disc formed from the stellar debris. Our TDE detection threshold can be lowered by a factor of similar to 2, which should allow a detection of similar to 700 TDEs by the end of the SRG survey.

Automated summary

This study presents the first sample of tidal disruption events (TDEs) discovered during the SRG all-sky survey. The X-ray spectra of these events are consistent with critical accretion onto black holes, with potential supercritical accretion in some cases. Optical brightening was observed in four TDEs, while the majority showed no optical activity. The study suggests that X-ray bright events constitute a minority of TDEs, with a detection threshold that can be lowered to potentially detect around 700 TDEs by the end of the survey.