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

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 (10 June–14 December 2020) 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 108 M⊙, 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 9 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 disk 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.

The candidate tidal disruption event AT2019fdr coincident with a high-energy neutrino

“…−33 eV and is among the softest X-ray spectra of all TDEs so far detected by SRG/eROSITA [96]. The best fit value of the equivalent hydrogen column density NH = 1.47 +2.…”

Discovery of ATLAS17jrp as an Optical-, X-Ray-, and Infrared-bright Tidal Disruption Event in a Star-forming Galaxy

“…Right panel: spectral indices of known TDE hosts (colored) and SDSS (gray) galaxies (French et al 2016;Graur et al 2018), i.e., Hα EW (sensitive to current star formation) versus Lick Hδ A absorption (sensitive to star formation over the past gigayear) for each galaxy. We have shown the hosts of known TDE candidates (French et al 2020;Sazonov et al 2021) properties to ATLAS17jrp (Jiang et al 2021a). In Figure 3, we show the distribution of these MIRONG in SF galaxies without preference for post-starburst galaxies, and therefore, they could fill the absence of TDEs in SF galaxies, part of which were missed by optical or X-ray surveys due to dust/gas obscuration, like MIRONG.…”

An Analytic, Fully Relativistic Framework for Tidal Disruption Event Streams in Schwarzschild Geometry

“…Dai et al 2018;. However, the all-sky X-ray survey carried out by SGR/eROSITA has recently discovered a sample of thirteen X-ray tidal disruption events, only four of which showed any optical activity (Sazonov et al 2021). We remark that there is a large parameter space available for TDEs which produce no unbound material during stream self-intersection, shown to the left of the dashed lines in Figure 12.…”

“…Dai et al 2018;. On the other hand, the SGR/eROSITA all-sky X-ray survey has recently identified a population of TDEs without any detectable optical emission (Sazonov et al 2021). These observations, among others, motivate a more comprehensive understanding of the TDE stream self-intersection process.…”