eROSITA-RU Tidal Disruption Events with Keck-I/LRIS: Sample Selection, Optical Properties, and Host Galaxy Demographics

We select seventy tidal disruption event (TDE) candidates among X-ray transients discovered during the eROSITA all-sky surveys in the Eastern Galactic hemisphere between December 2020 and February 2022 (eRASS1–5). We cross-match each X-ray source to a host galaxy in archival optical surveys using Bayesian likelihood-ratio techniques and obtain Keck/LRIS spectroscopy for all 70 host galaxies. Host properties are inferred through SED fitting with Prospector and emission line analysis with pPXF. We develop a robust classification scheme using X-ray and broad line luminosities, narrow-line ionization diagnostics, and optical variability to identify high-confidence TDEs, for which we analyze optical spectral features, light curve properties, and host galaxy demographics. Our final sample contains 52 TDEs with redshifts of $0.018 \leq z\leq0.714$, comprising 41 gold (high-confidence) and 11 silver (lower-confidence) events. The vast majority (93%) of gold TDEs are intrinsically brighter in the X-ray band, with $L_{\rm X,peak} > L_{\rm opt,peak}$. Among 23 events with detected optical flares, delayed X-ray peak is commonly observed. We identify transient spectral features in eight events, including six with prominent broad \ion{He}{II} $λ$4686 and/or H$α$ emission and two coronal-line TDEs. Host galaxy demographics reveal modest over-representation in green valley ($\times1.8$) and quiescent Balmer-strong ($\times5.3$) galaxies, significantly weaker than previous TDE samples, demonstrating greater diversity in star formation histories than previously recognized. Most TDE hosts exhibit suppressed star formation relative to the main sequence, consistent with X-ray selection biases against dusty star-forming galaxies.

💡 Research Summary

This paper presents a comprehensive analysis of 70 tidal disruption event (TDE) candidates discovered as X-ray transients by the eROSITA telescope during its all-sky surveys (eRASS1–5) in the Eastern Galactic Hemisphere. The study focuses on constructing a robust, spectroscopically confirmed sample to investigate the optical properties of the transients and the demographics of their host galaxies.

The initial sample was selected from highly variable X-ray sources in the eROSITA-RU catalog, applying filters to exclude Galactic objects and known active galactic nuclei (AGN). Each X-ray source was then cross-matched to a potential host galaxy in optical survey data (Pan-STARRS1 or DESI Legacy Surveys) using Bayesian likelihood-ratio techniques. Follow-up optical spectroscopy for all 70 host galaxy candidates was obtained with the Low-Resolution Imaging Spectrometer (LRIS) on the Keck I telescope.

The authors performed a detailed multi-wavelength analysis. Host galaxy properties—including stellar mass, star formation rate (SFR), and stellar population age—were derived by fitting the spectral energy distribution (SED) from UV to mid-infrared data using Prospector. Emission line analysis was conducted with pPXF. A novel, multi-criteria classification scheme was developed, combining X-ray-to-optical luminosity ratios, the presence of broad emission lines, narrow-line ionization diagnostics, and optical variability. This scheme was used to distinguish genuine TDEs from AGN and other transients, resulting in a final, high-quality sample of 52 TDEs at redshifts 0.018 ≤ z ≤ 0.714. This sample is subdivided into 41 “gold” (high-confidence) and 11 “silver” (lower-confidence) events.

Key findings include:

1. X-ray Dominance: The vast majority (93%) of gold-class TDEs are intrinsically brighter in X-rays than in the optical at peak ((L_{\rm X,peak} > L_{\rm opt,peak})), defining them as “X-ray loud,” in contrast to many optically selected TDEs.
2. Peak Delay: For the 23 events with detected optical flares, a delayed X-ray peak relative to the optical peak was commonly observed.
3. Spectral Diversity: Transient spectral features were identified in eight events, including six with prominent broad He II λ4686 and/or Hα emission, and two “coronal-line” TDEs.
4. Host Galaxy Demographics: The hosts show only a modest over-representation in green valley (×1.8) and quiescent Balmer-strong (×5.3) galaxies. This over-representation is significantly weaker than reported in previous, smaller TDE samples, indicating a greater diversity in host galaxy star-formation histories than previously recognized. Most hosts exhibit suppressed star formation relative to the main sequence, consistent with an X-ray selection bias against dusty, star-forming galaxies.

This work establishes a large, uniformly analyzed sample of X-ray-selected TDEs with detailed host galaxy characterization. It demonstrates systematic differences between X-ray and optically selected TDE populations, provides crucial constraints for TDE rate calculations and black hole demography studies, and offers new insights into the accretion physics of tidal disruption events.