The population of tidal disruption events discovered with eROSITA

Abstract

Aims. The Spectrum Roentgen Gamma (SRG) eROSITA all-sky survey marks the beginning of the data-rich era by conducting population studies of tidal disruption events (TDEs) and other rare X-ray transients. This paper presents a systematic study of X-ray-selected canonical TDEs discovered in the western Galactic hemisphere of the first two eROSITA all-sky surveys (eRASS1 and eRASS2) performed between Dec 2019 and Dec 2020. Methods. We compiled a TDE sample from the catalog of eROSITA's extragalactic transients and variables eRO-ExTra, which includes X-ray sources with a variability significance and fractional amplitude over four between eRASS1 and eRASS2, not associated with known active galactic nuclei (AGNs). Each X-ray source is associated with an optical counterpart from the Legacy Survey DR10 (LS10). Canonical TDEs were selected based on their X-ray light-curve properties (single flare or decline), soft X-ray spectra (Γ>3), and the absence of archival X-ray variability and AGN signatures in their host photometry and spectroscopy. Results. We present 31 X-ray-selected TDE candidates associated with optical counterparts with redshifts of 0.02< z <0.34 and luminosities of 5.7×10 41 < L X <5.3×10 44 ergs −1 in the 0.2−6.0 keV rest frame. The sample contains 30 canonical TDEs and one off-nuclear TDE candidate. The X-ray luminosity function derived from this sample is best fit by a double power law with a luminosity break at 10 44 ergs −1 , corresponding to the Eddington-limiting prediction. The result is in agreement with previous observational and theoretical estimates. This corresponds to a TDE volumetric rate of (2.3 −0.9 +1.2 ) × 10 −7 Mpc −3 yr −1 (≈1.2×10 −5 events per galaxy per year). The TDE host galaxies show a green-valley overdensity, as was previously found in X-ray and optical studies. In addition, 20%, 30%, and 15% of our X-ray-selected sample exhibit flares in the optical, mid-infrared (mid-IR), or radio bands, respectively. We discuss the differences between X-ray, optical, and mid-IR TDE populations and the origins of multiwavelength flares in the context of the obscuring envelope and stream-stream collision models. Finally, we highlight TDE subpopulations that are not included in the canonical sample and should be explored in the future.