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

TL;DR

We compile and analyze 70 eROSITA-detected TDE candidates with Keck/LRIS spectroscopy to derive host galaxy properties and robustly classify TDEs. Using Prospector SED fitting and pPXF emission-line analysis, we construct a gold sample of 41 high-confidence TDEs (and a silver sample of 11) across z ≈ 0.02–0.71, finding that 93% have peak X-ray luminosities exceeding their optical peaks. Optical flares are present in about half of the gold sample, with many events showing delayed X-ray peaks by months, and we identify eight TDE-associated spectral features, including six broad He II and/or Hα lines and two coronal-line TDEs. Host galaxies show modest overrepresentation in green valley and quiescent Balmer-strong systems, with most hosts lying below the star-forming main sequence, indicating diversity in star-formation histories and selection effects against dusty star-forming environments. Overall, this work delivers the largest, uniformly analyzed eROSITA TDE sub-sample to date and emphasizes the need to consider host demographics and multiwavelength timing when modeling TDE emission mechanisms and SMBH demographics.

Abstract

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.

Figures (20)

• Figure 1: Keck-I/LRIS optical spectrum of SRGe J004123.2$-$153705 (ID 2), a CrL-TDE.
• Figure 2: Top: Peak 0.3--2.3 keV X-ray luminosity versus broad H$\alpha$ luminosity. The solid line shows the best-fit relation from the corrected calibration of pulatova2025, and the shaded region indicates the intrinsic scatter of the correlation. Note that the H$\alpha$ luminosity of SRGe J234034.7+293400 (ID 69), is derived from DESI spectrum (see §\ref{['subsec:opt_spec']}), while others are all from Keck LRIS spectra. Bottom: $L_{\rm X, peak}$ versus [O III]$\lambda5007$ luminosity for the remaining sources. The shaded region shows the AGN locus Ueda_2015 calibrated in the eROSITA 0.3--2.3 keV band Khorunzhev2022, with the solid line indicating the median relation $\log(L_{[\mathrm{O\,III}]}/L_{\rm X}) = -1.99 \pm 0.63$. Left-pointing triangles denote sources with upper limits on [O$\;$III] line luminosity.
• Figure 3: BPT diagrams of host galaxies. The solid curves mark the theoretical maximum starburst demarcations of 2006MNRAS.372..961K, and the dashed curve in the left panel shows the empirical star-forming/composite boundary from 2003MNRAS.346.1055K.
• Figure 4: WHAN diagram of host galaxies. The horizontal lines at $\mathrm{EW}(\mathrm{H}\alpha)=6$ Å and $3$ Å indicate the sAGN/wAGN and wAGN/retired divisions, respectively, following 2011MNRAS.413.1687C.
• Figure 5: Spectral modeling of six objects with broad lines associated with the TDE. The observed LRIS data is shown in black. The orange line shows the Prospector-derived host model, and the red line shows the final continuum (polynomial fit plus scaled host model). The continuum-subtracted residuals are shown in teal. The individual Gaussian fits are shown as dashed curves, with vertical dotted lines marking their rest-frame wavelengths: purple for the Bowen blend, red for He ii$\lambda4686$, orange for H$\beta$, and blue for the broad component of H$\alpha$$\lambda6563$. The solid black and blue curves show the total models in the He ii and H$\alpha$ regions, respectively. Grey bands indicate masked telluric regions.