First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z < 0.6
Yao, Y.; Sunyaev, R.; Sazonov, S.; Bikmaev, I. F.; Gumerov, R. I.; Postnov, K. A.; Cherepashchuk, A. M.; Burenin, R.; Gilfanov, M.; Dodin, A. V.; Dodonov, S. N.; Belinski, A. A.; Semena, A.; Kotov, S. S.; Uskov, G.; Zaznobin, I.; Grokhovskaya, A. A.; van Velzen, S.; Kulkarni, S.; Medvedev, P.; Khorunzhev, G.; Lyapin, A.; Meshcheryakov, A.; Eselevich, M.; Zhuchkov, R. Ya
Russia, Germany, United States, Netherlands
Abstract
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° < l < 180° 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 ×103 to $10^8\, \mathrm{ M}_\odot$, 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 Lg/LX < 0.3 (Lg and LX 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) × 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 ~2, which should allow a detection of ~700 TDEs by the end of the SRG survey.