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

High Energy Astrophysic

Observation of a Very Massive Galaxy Cluster at z = 0.76 in the SRG/eROSITA All-Sky Survey

Abstract: The results of multiwavelength observations of the very massive galaxy cluster SRGe CL2305.2–2248 detected in X-rays during the first SRG/eROSITA all-sky survey are discussed. This galaxy cluster has also been detected previously in the millimeter band in the South Pole Telescope (SPT-CL J2305–2248) and Atacama Cosmology Telescope (ACT-CL J2305.1–2248) surveys through the observation of the Sunyaev–Zeldovich effect. The spectroscopic redshift has been measured at the 6-m BTA telescope, z=0.7573. In addition, deep photometric measurements of galaxies have been performed at the Russian–Turkish 1.5-m telescope (RTT-150). The mass of the cluster is estimated from the eROSITA data to be (Formula presented.). We show that this cluster is among several tens of the most massive clusters in the observable Universe and among only a few most massive galaxy clusters at z>0.6

First tidal disruption events discovered by SRG/eROSITA: X-ray/optical properties and X-ray luminosity function at z &lt; 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° < 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 108M⊙, 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