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arxiv: 2512.12480 · v1 · submitted 2025-12-13 · 🌌 astro-ph.HE · astro-ph.GA

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

Zirui Zhang , Yuhan Yao , Marat Gilfanov , Sergey Sazonov , Pavel Medvedev , Georgii Khorunzhev , Rashid Sunyaev , Vikram Ravi
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S. R. Kulkarni Jean Somalwar Ryan Chornock Ilfan Bikmaev Mark A. Gorbachev
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Pith reviewed 2026-05-16 22:13 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.GA
keywords tidal disruption eventseROSITAhost galaxy demographicsX-ray transientsoptical spectroscopygreen valley galaxiesstar formation historyKeck observations
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The pith

X-ray selected TDEs show only modest over-representation in green valley and quiescent Balmer-strong galaxies.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper selects 70 TDE candidates from eROSITA X-ray transients in the Eastern Galactic hemisphere and secures Keck/LRIS spectra for every host galaxy. It applies a classification scheme based on X-ray and broad-line luminosities, narrow-line diagnostics, and optical variability to isolate 52 events as TDEs. The resulting host demographics display only a factor of 1.8 excess in green-valley galaxies and a factor of 5.3 excess in quiescent Balmer-strong galaxies, both weaker than earlier optically selected samples. This indicates that TDE hosts span a wider range of star-formation histories than previously inferred from optical surveys alone.

Core claim

A sample of 52 eROSITA X-ray TDEs, vetted with multi-wavelength diagnostics, shows host galaxies with modest over-representation in green-valley (×1.8) and quiescent Balmer-strong (×5.3) systems, weaker than prior TDE samples, together with suppressed star formation relative to the main sequence and a predominance of X-ray-luminous events.

What carries the argument

Classification scheme that combines X-ray and broad-line luminosities, narrow-line ionization diagnostics, and optical variability to separate true TDEs from supernovae and AGN contaminants.

If this is right

  • 93 percent of high-confidence TDEs are intrinsically brighter in X-rays than in the optical at peak.
  • Delayed X-ray peaks relative to optical flares are common among events with detected optical emission.
  • Transient broad He II and H-alpha features appear in a subset of events, including two coronal-line cases.
  • TDE hosts lie below the star-forming main sequence, consistent with selection against dusty star-forming galaxies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • X-ray selection may uncover TDEs in a broader range of galaxy evolutionary states than optical selection alone.
  • The weaker demographic bias suggests that optical surveys previously missed TDEs in more typical star-forming hosts.
  • Combining eROSITA-style X-ray triggers with optical spectroscopy could map how TDE rates depend on galaxy quenching stage.

Load-bearing premise

The multi-wavelength classification scheme correctly identifies genuine TDEs and excludes most supernovae and active galactic nuclei.

What would settle it

Follow-up observations that reclassify a substantial fraction of the gold-sample events as supernovae or AGN variability instead of TDEs.

Figures

Figures reproduced from arXiv: 2512.12480 by Georgii Khorunzhev, Ilfan Bikmaev, Jean Somalwar, Marat Gilfanov, Mark A. Gorbachev, Pavel Medvedev, Rashid Sunyaev, Ryan Chornock, Sergey Sazonov, S. R. Kulkarni, Vikram Ravi, Yuhan Yao, Zirui Zhang.

Figure 1
Figure 1. Figure 1: Keck-I/LRIS optical spectrum of SRGe J004123.2−153705 (ID 2), a CrL-TDE. After examining all LRIS spectra of our candidates, we identified two objects that exhibit high-ionization coronal lines: SRGe J004123.2-153705 (ID 2) and SRGe J201138.9-210935 (ID 63). The optical counterpart of SRGe J201138.9-210935 (ID 63), TDE 2021qth/ZTF21abhrchb, has been previously classified as a CrL-TDE by Yao et al. (2023). … view at source ↗
Figure 3
Figure 3. Figure 3: BPT diagrams of host galaxies. The solid curves mark the theoretical maximum starburst demarcations of Kewley et al. (2006), and the dashed curve in the left panel shows the empirical star-forming/composite boundary from Kauffmann et al. (2003a) [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: WHAN diagram of host galaxies. The horizontal lines at EW(Hα) = 6 Å and 3 Å indicate the sAGN/wAGN and wAGN/retired divisions, respectively, following Cid Fernandes et al. (2011). 021812 (ID 15) exhibits, in addition to broad Balmer lines, broad Hβ and broad Mg II λλ2796, 2803 emission which is rarely de￾tected in known TDEs (Hung et al. 2019), favoring a BLAGN interpretation. In contrast, SRGe J175023.7+7… view at source ↗
Figure 5
Figure 5. 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… view at source ↗
Figure 6
Figure 6. Figure 6: shows the distribution of our gold sample in the Lopt,peak versus LX,peak plane. Note that peak X-ray luminosity LX,peak is estimated from the observed peak flux (fX,peak in Ta￾bles 1 and 2) captured by the 6-month-cadence eROSITA survey 5 Note that in our previous work (Sazonov et al. 2021), we considered only the median forced photometry flux uncertainty. However, forced photometry uncertainties can be u… view at source ↗
Figure 7
Figure 7. Figure 7: Time difference between the peak X-ray and peak optical emis￾sion. The distribution is shown as a histogram, with individual measure￾ments marked along the x-axis. Our sample demonstrates that delayed X-ray peak is common among X-ray selected TDEs. and is therefore likely fainter than the true intrinsic X-ray peak. Nevertheless, 38 of 41 events have Lopt,peak < LX,peak, demon￾strating that the vast majorit… view at source ↗
Figure 8
Figure 8. Figure 8: Broad emission line luminosity evolution for six TDEs with sig￾nificant broad lines detected in our Keck spectra (see §4.3.4). Circles and diamonds represent the fitted luminosity of He II λ4686 (broad or shifted components) and Hα (broad + shifted components) transient lines, respectively. Downward triangles indicate 3σ upper limits. 5.2.1. Broad Hα without He (ID 41) SRGe J153503.3+455054 (ID 41) exhibit… view at source ↗
Figure 9
Figure 9. Figure 9: Color–mass diagram for the final TDE host sample. The light￾green band indicates the green-valley region defined by Schawinski et al. (2014). The darker-green band shows our empirically derived GV definition based on our SDSS comparison sample. Background contours are derived from our SDSS comparison galaxies (§4.4), shown at the 6.7–93.3% density levels. Galaxy colors provide a simple diagnostic of star f… view at source ↗
Figure 10
Figure 10. Figure 10: Lick HδA absorption index versus Hα emission equivalent width for our TDE host sample. The background shows the SDSS com￾parison sample with contours at σ-equivalent probability levels (6.7– 93.3%). The solid and dashed boxes mark the E+A and QBS regions, respectively. To avoid label overlap, our gold and silver hosts near the origin are not individually labeled since many are clustered there. is not avai… view at source ↗
Figure 11
Figure 11. Figure 11: shows the SFR averaged over the past 100 Myr (SFR100) versus stellar mass from our Prospector fits. The back￾ground distribution shows the volume-limited SDSS compari￾son sample, and the solid black line indicates the star-forming main sequence from Popesso et al. (2023). Following Csizi et al. (2024), we split into two redshift bins (0.0 ≤ z < 0.2 and 0.2 ≤ z < 0.8), which affects the main sequence. Quan… view at source ↗
read the original 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} $\lambda$4686 and/or H$\alpha$ 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports the selection of 70 TDE candidates from eROSITA X-ray transients (eRASS1-5) in the Eastern Galactic hemisphere, cross-matched to hosts via Bayesian likelihood ratios and followed up with Keck/LRIS spectroscopy. A multi-criteria classification scheme based on X-ray/optical luminosities, broad-line strengths, narrow-line BPT diagnostics, and optical variability is used to define 52 TDEs (41 gold high-confidence, 11 silver). The paper analyzes optical spectral features, light-curve properties, and host demographics via Prospector SED fitting and pPXF line analysis, reporting that gold TDEs are mostly X-ray bright, often show delayed X-ray peaks, and exhibit modest over-representation in green-valley (×1.8) and quiescent Balmer-strong (×5.3) galaxies relative to the field population—factors weaker than in prior samples, implying greater diversity in star-formation histories.

Significance. If the classification holds, the work delivers one of the largest spectroscopically confirmed X-ray-selected TDE samples to date, directly addressing selection biases against dusty star-forming hosts and showing that TDE hosts span a broader range of star-formation histories than previously inferred from optically selected samples. This has implications for TDE rate calculations and for using TDEs as probes of supermassive black hole demographics across galaxy types.

major comments (1)
  1. [Classification scheme] The headline demographic results (green-valley over-representation ×1.8 and quiescent Balmer-strong ×5.3) are derived exclusively from the 41 gold TDEs. The classification scheme (X-ray vs. optical luminosity, broad-line luminosities, narrow-line ionization diagnostics, and optical variability) is asserted to be robust, yet no quantitative false-positive rate, purity estimate, or host-type-dependent contamination model is supplied. Changing-look AGN and certain nuclear transients can satisfy the same flare + broad-line + variability criteria while residing in green-valley or post-starburst hosts; even modest contamination would shift the reported factors and the 'greater diversity' conclusion.
minor comments (2)
  1. [Abstract] The abstract states that the over-representation factors are 'significantly weaker than previous TDE samples' but provides neither citations to the specific prior works nor the numerical factors being compared.
  2. [Classification scheme] Exact numerical thresholds used to separate gold from silver events (e.g., luminosity ratios, line-strength cuts, variability significance) are not tabulated or stated explicitly, limiting reproducibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We appreciate the referee's careful review of our manuscript on eROSITA-selected TDEs. We address the major comment on the classification scheme below.

read point-by-point responses

  1. Referee: [Classification scheme] The headline demographic results (green-valley over-representation ×1.8 and quiescent Balmer-strong ×5.3) are derived exclusively from the 41 gold TDEs. The classification scheme (X-ray vs. optical luminosity, broad-line luminosities, narrow-line ionization diagnostics, and optical variability) is asserted to be robust, yet no quantitative false-positive rate, purity estimate, or host-type-dependent contamination model is supplied. Changing-look AGN and certain nuclear transients can satisfy the same flare + broad-line + variability criteria while residing in green-valley or post-starburst hosts; even modest contamination would shift the reported factors and the 'greater diversity' conclusion.

    Authors: We thank the referee for this insightful comment. Our classification scheme combines multiple independent criteria to define the gold sample, with particular emphasis on X-ray dominance (93% of gold TDEs satisfy L_X,peak > L_opt,peak) and narrow-line BPT diagnostics to rule out AGN. We agree that a quantitative false-positive rate is not provided in the current version. In the revised manuscript, we will include an expanded discussion of potential contaminants such as changing-look AGN, providing an order-of-magnitude estimate of their contribution based on literature rates and the eROSITA survey characteristics. We will also clarify that while a full host-type-dependent contamination model would be ideal, the weaker over-representation factors compared to prior samples suggest that contamination is unlikely to drive the observed greater diversity in star formation histories. We will note this as a caveat in the text. revision: partial

Circularity Check

0 steps flagged

No significant circularity; purely observational classification and demographics

full rationale

The paper performs candidate selection from eROSITA X-ray transients, cross-matching to hosts, Keck spectroscopy, SED fitting via Prospector, line analysis via pPXF, and a multi-criteria classification (X-ray/optical luminosities, broad-line strengths, BPT diagnostics, variability) to define gold/silver TDEs. Demographics (green-valley ×1.8, quiescent Balmer-strong ×5.3) are then computed directly on the resulting 41 gold events against external surveys. No equations, fitted parameters renamed as predictions, self-definitional loops, or load-bearing self-citations appear; the classification scheme is presented as a data-driven filter without reducing to its own outputs by construction. This is a standard observational pipeline with independent external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No free parameters, axioms, or invented entities are introduced; the work applies standard astrophysical analysis pipelines and classification criteria to new observational data.

pith-pipeline@v0.9.0 · 5698 in / 1244 out tokens · 70685 ms · 2026-05-16T22:13:51.068122+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

4 extracted references · 4 canonical work pages · 1 internal anchor

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    For ID 47, since the optical flare was detected during ZTF reference image building, we perform baseline correction following Yao et al

    are beyond the time range of the x-axis. For ID 47, since the optical flare was detected during ZTF reference image building, we perform baseline correction following Yao et al. (2019). Figures A.7 and A.8 show theProspectorSED fitting re- sults of the host galaxies of our gold and silver samples. Appendix B: Supplementary Tables Table B.1 shows the prope...

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    TDE-H+He 11 He IIλ4686 4690.0±2.67 39.45±7.52 30132±3366 2420±674 Hαbroad 6588.2±110.4 20.20±4.29 12029±4033 1018±209 Hαrest 6567.1±17.2 3.82±2.35 1909±1380 193±119 Hαblue 6472.6±20.8 2.69±2.02 2238±1289 127±105 33 He IIλ4686 4690.00±0.85 15.18±6.32 22598±4185 604.2±251.5 Hαbroad 6562.8±126.5 14.90±4.98 9265±3765 450±151 Hαrest 6567.1±19.7 3.67±1.89 1855±...

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  4. [4]

    Given the non-detection of Hα, we classify this object as TDE-He

    TDE-He 197 He IIλ4686 4515.7±12.5 69.97±13.48 24262±7809 966.9±179.2 Hβbroad 4861.3 5.73±2.22 1595±607 79.88±32.15 Hαbroad — — —<48.81 Notes.For SRGe J235453.0+421711 (ID 70), the skewness of He IIλ4686 line is−0.36±0.29. Given the non-detection of Hα, we classify this object as TDE-He. While a Gaussian centered on Hβis favored by BIC (∆BIC=11.42>10), we ...

    work page 2020