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

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Gilfanov,  M.
High Energy Astrophysics, MPI for Astrophysics, Max Planck Society;

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Sunyaev,  R.
High Energy Astrophysics, MPI for Astrophysics, Max Planck Society;

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Citation

Sazonov, S., Gilfanov, M., Medvedev, P., Yao, Y., Khorunzhev, G., Semena, A., et al. (2021). First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z < 0.6. Monthly Notices of the Royal Astronomical Society, 508(3), 3820-3847. doi:10.1093/mnras/stab2843.


Cite as: http://hdl.handle.net/21.11116/0000-0009-C9B9-4
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 ×10<sup>3</sup> to 10<sup>8</sup>M<sub>⊙</sub>⁠, 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 Lsub>g</sub>/L<sub>X</sub> < 0.3 (L<sub>g</sub> and L<sub>X</sub> 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<sup>−5</sup> 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.