SRG/eROSITA prospects for the detection of stellar tidal disruption flares

Abstract

We discuss the potential of the eROSITA telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory to detect stellar tidal disruption events (TDE) during its 4-year all-sky survey. These events are expected to reveal themselves as luminous flares of UV/soft X-ray emission associated with the centres of previously non-active galaxies and fading by few orders of magnitude on time-scales of several months to years. Given that eROSITA will complete an all-sky survey every 6 months and a total of 8 such scans will be performed over the course of the mission, we propose to distinguish TDEs from other X-ray transients using two criteria: I) large (more than a factor of 10) X-ray variation between two subsequent 6-month scans and ii) soft X-ray spectrum. The expected number of TDE candidates is $\sim 10^3$ per scan (with most of the events being new discoveries in a given scan), so that a total of several thousand TDE candidates could be found during the 4-year survey. The actual number may significantly differ from this estimate, since it is based on just a few TDEs observed so far. The eROSITA all-sky survey is expected to be nearly equally sensitive to TDEs occurring near supermassive black holes (SMBH) of mass between $\sim 10^6$ and $\sim 10^7 M_\odot$ and will thus provide a unique census of quiescent SMBHs and associated nuclear stellar cusps in the local Universe ($z\lesssim 0.15$). Information on TDE candidates will be available within a day after their detection and localization by eROSITA, making possible follow-up observations that may reveal peculiar types of TDEs.