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REVIEW 4 major objections 9 minor 63 references

New TeV-emitting BL Lac candidates from the eROSITA X-ray survey

T0 review · 4 major / 9 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read The deepest wide-area soft-X-ray survey to date yields 121 new TeV-emitting BL Lac blazar candidates.

desk verdict Useful eROSITA-based HBL candidate list that deserves refereeing, but the paper as written cannot be reproduced because the candidate count and the alpha_IR-X cut are stated inconsistently. read the letter →

arxiv 2501.12520 v3 pith:CUP3N3EP submitted 2025-01-21 astro-ph.HE

Cassidy Metzger , Andrea Gokus , Manel Errando This is my paper · ORCID
classification astro-ph.HE
keywords BLLacertaeobjectsTeVgamma-rayastronomyeROSITAX-raysurveysblazarcandidateshigh-synchrotron-peakedblazarsWISEinfraredcolorsradio-to-infraredspectralindex
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that the deepest wide-area soft-X-ray survey yet released, the eROSITA eRASS1 catalog, can substantially expand the known population of TeV-emitting BL Lac blazars. Cross-matching infrared and X-ray colors plus archival radio data yields 121 high-synchrotron-peaked BL Lac candidates, including 23 objects too faint for the earlier ROSAT survey and 8 that had never been flagged as blazars. The new piece is a radio-to-infrared spectral index cut that removes quasar-like objects whose infrared and X-ray properties mimic genuine TeV blazars. If pointed TeV observations confirm the candidates, the list gives current and future Cherenkov telescopes a ready queue of targets ranked by X-ray brightness.

What carries the argument

The machinery is a nine-step filtering funnel applied to roughly 747 million AllWISE sources, of which 121 survive to the final list. The load-bearing new ingredient is the radio-to-infrared effective spectral index $\alpha_{\rm R-IR} = -\log(F_R/F_{3.4\,\mu\rm m})/\log(\nu_R/\nu_{3.4\,\mu\rm m})$, required to be at most 0.43: in the plane of $\alpha_{\rm R-IR}$ versus $\alpha_{\rm IR-X}$, flat-spectrum radio quasars and BL Lacs separate, and the cut rejects quasars whose synchrotron peak is in the infrared but whose hard X-ray spectrum mimics an HBL. The funnel also uses the WISE 3.4–4.6–12 $\mu$m color-color strip calibrated on the 56 known TeV HBLs (distance no more than 0.21 from the line $y=0.84x-1.05$), a 5″ positional match to eRASS1, $\alpha_{\rm IR-X}\ge 0$, non-extended X-ray emission, $|b|\ge 10^\circ$, and a radio counterpart. The surviving sources are ranked by X-ray flux because the paper treats the 0.2–2.3 keV flux as the practical predictor of TeV detectability.

What would settle it

Pointed TeV observations of the X-ray-brightest candidates: if most show no >0.1 TeV emission while optical spectra show broad quasar emission lines, the radio-to-infrared cut is not separating as assumed. A cheaper check is spectra of the eight new candidates, where broad lines would mark them as quasars that slipped through.

Watch

Extended reading notes

Core claim

The central claim is a ranked candidate catalog rather than a detection: 121 high-frequency-peaked BL Lac objects selected from the eRASS1 X-ray source list, AllWISE infrared magnitudes, and NVSS/SUMSS/FIRST radio catalogs, ordered by their 0.2–2.3 keV flux because X-ray and TeV fluxes are empirically correlated in this class. The selection requires the WISE color-color strip occupied by gamma-ray blazars, an infrared-to-X-ray effective spectral index $\alpha_{\rm IR-X}\ge 0$, point-like X-ray morphology, Galactic latitude $|b|\ge 10^\circ$, a radio counterpart within 5″, and the new radio-to-infrared cutoff $\alpha_{\rm R-IR}\le 0.43$. Of the 121 candidates, 53 are already identified as BL Lacs in ROMA-BZCAT, 108 overlap the 3HSP catalog, 23 have no ROSAT counterpart, and 8 are new blazar candidates. The paper's case is that the depth and energy coverage of eRASS1, together with the radio-IR cut, is what brings these objects to the surface.

Load-bearing premise

The whole list rests on the assumption that the radio-versus-infrared brightness cut, tuned to known blazars and quasars, separates faint unknown sources of the two kinds just as cleanly; no independent sample is used to measure how many quasar-like objects still pass.

Editorial extensions

If this is right

  • The 121-candidate list immediately supplies observation targets for VERITAS, MAGIC, HESS, and LST-1, with the X-ray-ranked tables telling observers which northern and southern sources to point at first.
  • Because 23 candidates were below the ROSAT detection threshold, the known TeV HBL census of 56 objects is incomplete in exactly the flux range eRASS1 now covers.
  • Eight candidates with no prior blazar association show that blazar catalogs built on shallower X-ray surveys can miss genuine members of the class.
  • The same $\alpha_{\rm R-IR}\le 0.43$ cut can be reused on future eROSITA data releases to extend the search to the rest of the sky and to deeper exposure.
  • The 31 extended-X-ray rejects and 18 radio-undetected rejects are tabulated as secondary follow-up targets, including possible BL Lacs in cluster cores.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: the $(\alpha_{\rm R-IR}, \alpha_{\rm IR-X})$ plane could become a standard two-index diagnostic for classifying blazar SEDs in any future survey combining radio, infrared, and X-ray data.
  • Editorial inference: if even a few of the X-ray-brightest candidates are confirmed at TeV energies, it would validate the X-ray-flux-to-TeV-flux correlation in a regime below the ROSAT era and strengthen the case that X-ray depth is the most efficient path to new TeV blazars.
  • Editorial inference: optical spectroscopy of the eight new candidates is the quickest falsification check, since genuine BL Lacs have featureless optical continua while quasars show broad emission lines.
  • Editorial inference: applying the $\alpha_{\rm R-IR}$ cut to the full set of 56 known TeV HBLs, not just the 21 in the eRASS1 footprint, would give a quantitative false-negative rate for the cut, a number the paper does not report.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 9 minor

Summary. The manuscript presents a search for new TeV-emitting high-synchrotron-peaked BL Lac (HBL) candidates built on the eRASS1 X-ray catalog. The selection pipeline (Section 3, criteria I-IX and Figure 3) requires WISE detections with magnitude limits and colors near the locus of the 56 known TeV HBLs (a color box, a 0.21-mag distance to the fitted line, and a brightness cut), an X-ray counterpart in eRASS1 within 5 arcseconds, a point-like X-ray morphology, |b| ≥ 10 degrees, a radio counterpart in NVSS/FIRST/SUMSS/RADIO, and effective spectral-index cuts in the αR-IR versus αIR-X plane (αR-IR ≤ 0.43, and an αIR-X bound that the text states inconsistently). The final list is ranked by 0.2-2.3 keV X-ray flux for TeV follow-up. The paper claims 121 candidates (with 23 lacking ROSAT 2RXS counterparts and 11 not previously identified as blazars in 4LAC/BZCAT/3HSP, 8 of which are genuinely new), but the figure and table captions state 135 objects. The written pipeline, the figures, and the tables disagree on several counts, so the exact algorithm and the candidate list must be reconciled before the central claim can be verified.

Significance. This is a candidate-list paper with real value if the list is clean: only 56 TeV HBLs are currently known, so a flux-ranked sample of roughly 120 new candidates, including 23 objects below the ROSAT sensitivity and 8 objects not previously classified as blazars, is a useful resource for IACT follow-up with H.E.S.S., VERITAS, MAGIC, and CTAO. The paper earns credit for shipping the full selection code publicly (GitHub), stating the nine criteria explicitly, and providing falsifiable predictions that pointed TeV observations can confirm or reject. The radio-to-infrared spectral-index criterion (αR-IR ≤ 0.43) is a sensible, physically motivated addition to the established WISE-color plus X-ray selection of Massaro et al. (2011, 2013), Arsioli et al. (2015), and Chang et al. (2017), and the cross-checks against 3HSP, 4LAC, BZCAT, 2FHL, 3FHL, and 1CGH give useful context on how much of the list is genuinely new. The main risk is that the thresholds are calibrated on the known TeV HBL sample without a quantified contamination estimate, so the classification purity of the list is not yet demonstrated.

major comments (4)
  1. [Section 2 vs. Section 3 criterion V, Figure 2, Table A1] Section 2 states the infrared-to-X-ray selection as αIR-X ≥ 0 and asserts that four known TeV HBLs in the eRASS1 footprint (PKS 1440-389, 1ES 1215+303, TXS 1515-273, PKS 0301-243) have αIR-X < 0, and Section 3's narrative text repeats 'αIR-X ≥ 0 are selected (V)'. Enumerated criterion V and the Figure 2 caption, however, require αIR-X ≤ 1.0, and Table A1 lists exactly these four sources with αIR-X = 1.05-1.18, i.e., greater than 1, not less than 0. The candidate table itself corroborates the ≤ 1.0 version (all listed αIR-X values are ≤ 1.00). The Section 2 discussion therefore contains a sign error and a wrong cut direction, and because a reader implementing the stated ≥ 0 cut would retain the four known TeV HBLs and obtain a different candidate list, the published algorithm does not uniquely determine the result.
  2. [Abstract, Section 4, Section 5 vs. Figure 3, Table 1, Figure 4 captions] The headline count is stated inconsistently: the abstract, Section 4 ('We have identified 121 TeV HBL Candidates'), and Section 5 ('121 promising objects') report 121 candidates, whereas the Figure 3 caption ('The resulting outcome of 135 objects'), the Table 1 caption ('The complete list of 135 objects'), and the Figure 4 caption ('The 135 candidate sources identified in our study') report 135. Because the candidate list is the central result of the paper, the authors must determine which number the public code and the machine-readable table actually yield, make every statement and caption agree with it, and re-check the derived quantities that depend on the total (e.g., the 19% ROSAT-undetected fraction in Section 5).
  3. [Section 3, Figure 3, Section 4, Table A3] Figure 3 does not describe the same pipeline as criteria I-IX. The flow chart contains a box labeled 'External process to apply infrared cuts' (between 137 and 121 sources) that is never defined in the text, a TeVCAT-exclusion step that is absent from the criteria list, and an 'is extragalactic' condition folded into the extended-source filter that likewise has no written criterion. The step-by-step counts also disagree with the tables and text: Section 4 says 18 sources were discarded for lacking a radio counterpart, while Table A3 lists 36 entries and Figure 3 removes 36 (177→141); and Figure 3 removes 41 sources at the 'not extended and extragalactic' step (218→177) while Table A2 lists only 31 extended sources. The criteria list, Figure 3, and the appendix tables must be brought into agreement so that the selection can be reproduced from the paper alone.
  4. [Section 3, criterion IX, Figure 4] The radio-to-infrared cut αR-IR ≤ 0.43 is calibrated on the distribution of known TeV HBLs, ROMA-BZCAT BL Lacs, and FSRQs shown in Figure 4, but the paper provides no quantitative estimate of how many non-HBL sources pass the full set of criteria I-IX. Since the central claim is that the final list consists of TeV-emitting HBL candidates, the contamination rate directly affects the usefulness of the list for follow-up planning. I request a quantitative contamination test — for example, running the complete selection on the spectroscopically confirmed FSRQ population from ROMA-BZCAT or 4LAC and reporting the pass rate, and stating how many final candidates have spectroscopic classifications inconsistent with HBLs — in addition to the cross-checks against 3HSP, BZCAT, and 4LAC already provided in Section 5.
minor comments (9)
  1. [Section 5 vs. Table 4] The text refers to 'THC J0825+016', but Table 4 lists J0824+016; the two designations must be aligned.
  2. [Table 3, row J0244-583] The listed αIR-X = 0.24 disagrees with the value 0.75 in Table 1 for the same source with identical fluxes; one of the two entries is a typo.
  3. [Abstract vs. Section 5] The abstract's '11 that have not been previously associated with blazars' should be qualified, since Section 5 explains that 3 of the 11 appear in BROS, KDEBLLACS/WIBRaLS2, and the eROSITA blazar catalog, leaving 8 genuinely new candidates.
  4. [Section 4, first paragraph] 'a radio counterpart could no be identified' contains a typo ('no' should be 'not').
  5. [Figure 2 caption] 'the our selection criterion' contains a duplicated article.
  6. [Equation 2 and Table 1 notes] SUMSS fluxes are at 843 MHz while NVSS and FIRST fluxes are at 1.4 GHz, and the text says the radio flux is 'preferentially measured at νR = 1.4 GHz' without stating whether νR is adjusted for SUMSS-only sources; this should be stated explicitly, as a fixed νR = 1.4 GHz introduces a small systematic offset in αR-IR for those objects.
  7. [Section 4, coincident-rate estimate] The argument that the eRASS1 chance-coincidence rate follows from the eFEDS rate with 'approximately 10 times' lower exposure should be made explicit (the rate scales with source density, not exposure directly), so that the claim of fewer than one expected spurious candidate is checkable.
  8. [Section 2, criterion III] The 'distance of 0.21 from the best-fit line' is not defined; please state whether it is the perpendicular Euclidean distance in the color-color plane.
  9. [Section 5, Hämmerich et al. (in prep)] The statement that three candidates appear in the eROSITA blazar catalog relies on 'Hämmerich et al. (in prep)'; if that catalog is not yet public, this should be flagged or replaced with an available reference.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the 121-candidate list is a supervised selection from independent catalogs, not a fitted quantity; internal count and cut inconsistencies affect reproducibility but not circularity.

full rationale

The paper's selection pipeline is transparently supervised: the WISE color line y=0.84x-1.05, the 0.21 distance tolerance, the magnitude limits, the alpha_IR-X cut, and the alpha_R-IR <= 0.43 cut are all fit to the 56 known TeV HBLs (21 in the eRASS1 footprint) and to the BL Lac/FSRQ populations. The output, however, is a new list of objects selected from independent eRASS1, WISE, NVSS/SUMSS/FIRST catalogs; the identities, X-ray fluxes, and ranking are not the fitted parameters themselves. The paper labels the output 'candidates' and does not claim a confirmed TeV detection, so the derivation is a supervised search rather than a circular prediction. The only self-citation (Errando & Saito 2023) is a background reference on IACT sensitivity in the introduction and is not load-bearing. Separate internal inconsistencies—121 vs 135 counts between the abstract/Section 4 and Figures 3/4/Table 1, the alpha_IR-X cut stated as >= 0 in Section 2 but <= 1.0 in criterion V and Figure 2, and the undefined 'External process to apply infrared cuts' in Figure 3—are reproducibility and correctness concerns, not circularity. The central claim is therefore not equivalent to its inputs, and no circular step can be exhibited.

Assumptions & free parameters 9 free parameters · 6 assumptions · 0 invented entities

The central result relies on a supervised selection: thresholds are fit to known TeV HBL and FSRQ populations, then applied to eRASS1, WISE, and radio data. No new physical entities are postulated. All free parameters are catalog-selection thresholds rather than physical constants.

free parameters (9)
  • WISE color-color line slope = 0.84
    Best fit to the 56 known TeV HBLs in Figure 1; used as the center of the infrared color selection band.
  • WISE color-color line intercept = -1.05
    Same best fit; together with the slope it defines the selection line.
  • Maximum distance from infrared color line = 0.21
    Equal to the largest observed distance of the 56 known TeV HBLs from the fitted line; used as the selection tolerance in criterion III.
  • WISE magnitude limits = M3.4 < 14.3, M4.6 < 13.8, M12 < 12.2
    Chosen because all 56 known TeV HBLs are brighter than these values; criterion I.
  • WISE color box boundaries = 1.25 < M4.6-M12 < 2.4, 0.05 < M3.4-M4.6 < 0.9
    Adopted from the gamma-ray blazar region in Massaro et al. and used as criterion II.
  • alpha_IR-X threshold = >=0 (text) or <=1.0 (criterion V and Figure 2)
    The paper states both values in different places; the threshold is intended to retain known TeV HBLs while limiting candidates.
  • alpha_R-IR threshold = 0.43
    Chosen from the separation between BL Lacs and FSRQs in Figure 4; criterion IX.
  • X-ray match radius = 5 arcsec
    Set to the average eRASS1 positional error; criterion IV.
  • Galactic latitude cut = 10 degrees
    Chosen to reduce source contamination near the Galactic plane; criterion VII.
assumptions (6)
  • domain assumption The 56 sources in TeVCAT classified as HBLs are correctly classified and representative of the TeV-emitting BL Lac population.
    Used as the training sample for every selection threshold in Section 2 and Figure 1; if this sample is biased, the criteria inherit the bias.
  • domain assumption eRASS1 astrometric and photometric measurements are reliable to the quoted positional errors and fluxes.
    All X-ray counterparts and alpha_IR-X values depend on eRASS1; Section 3 criterion IV assumes that a 5 arcsecond matching radius is adequate.
  • domain assumption The WISE gamma-ray blazar strip defined by Massaro et al. applies to TeV HBLs.
    The infrared color box in criterion II is taken from earlier blazar catalogs and is assumed to contain TeV HBLs.
  • domain assumption The radio-to-infrared spectral index alpha_R-IR separates BL Lacs from FSRQs as shown in Figure 4.
    The new alpha_R-IR <= 0.43 cut rests on this empirical separation; no independent validation set is provided.
  • domain assumption X-ray flux correlates with TeV flux for HBLs, so ranking by eRASS1 flux prioritizes TeV-detectable sources.
    Used in Section 4 to build the 'most promising' target lists; based on cited literature (Fossati et al. 2008).
  • domain assumption Radio catalogs (NVSS, FIRST, SUMSS, RADIO) are deep enough that objects without a listed counterpart are genuinely radio-weak.
    Criterion VIII excludes 18 objects without radio counterparts; if those catalogs are incomplete at the relevant flux levels, real HBLs are lost.

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Cite this review

Pith. "Pith review of New TeV-emitting BL Lac candidates from the eROSITA X-ray survey." pith.science (2026). https://pith.science/paper/CUP3N3EP

@misc{pith2026250112520,
  author       = {Pith},
  title        = {Pith review of: New TeV-emitting BL Lac candidates from the eROSITA X-ray survey},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CUP3N3EP}},
  note         = {Machine review of arXiv:2501.12520}
}
read the original abstract

TeV-emitting BL Lac type blazars represent the extreme end of the blazar population. They are characterized by relatively weak jets and radiatively inefficient accretion disks. Particles accelerated in these jets experience fewer radiative losses, allowing them to reach energies beyond the TeV scale and produce TeV gamma-ray emission. The study of TeV blazars is constrained by the limited number of known sources in this category. Currently, only 56 high synchrotron-peaked BL Lacs have been detected at energies above 0.1 TeV. Searches for TeV emission from BL Lacs typically target sources with bright X-ray emission and a synchrotron peak at or above 1 keV. The recently released eRASS catalog by the eROSITA collaboration, which covers half of the sky, represents the deepest X-ray survey in the soft X-ray band to date. Utilizing the eROSITA survey, combined with infrared data from WISE and archival radio observations, we have identified 121 TeV-emitting blazar candidates. Our search introduces selection criteria based on the radio to infrared that remove quasar-like objects that have similar infrared spectra and X-ray fluxes as TeV-emitting BL Lacs. In our search, we find 23 objects that had not been detected in the ROSAT X-ray survey and 11 that have not been previously associated with blazars. The candidates resulting from our search are suitable for follow-up observations with currently operating imaging atmospheric Cherenkov telescopes, as well as future facilities like the CTAO Observatory.

Figures

Figures reproduced from arXiv: 2501.12520 by the authors.

Figure 1
Figure 1. The WISE 3.4–4.6–12 µm color-color plot for the 56 TBLs selected (red crosses). The dashed line corre￾sponds to the regression line described in Section 2. The black dashed rectangle corresponds to the maximum allowed separation (0.21) of the known TeV HBLs from this line. The turquoise points correspond to all WISEA sources found within the eROSITA footprint. emitting BL Lacs can further be separated from flat spec… view at source ↗
Figure 3
Figure 3. Description of the algortihm followed to identify TeV-emitting HBL candidates. The input surveys WISEA and eRASS1 are shown at the top in light blue. The resulting outcome of 135 objects is shown at the bottom, also in light blue. The number of objects that make it through each stage of filtering is shown in red boxes. Green boxes indicate parameters that are calculated based on catalog inputs. Based on the infrared… view at source ↗
Figure 4
Figure 4. The ratio between the radio and IR fluxes, nor￾malized by the ratios of their frequencies, plotted against αIR-X defined in Equation 1. BL Lac objects in ROMA￾BZCAT are displayed as yellow triangles, while FSRQs are displayed as blue triangles. The 21 TeV-detected HBLs within the eROSITA footprint are shown as unfilled red tri￾angles, while the ones designated as extreme HBLs (eHBLs) in TeVCat are shown as filled re… view at source ↗
Figures from the paper (3 more)
Figure 5
Figure 5. Figure 5: Distribution of X-ray fluxes for the TeV candidate blazars in different bands from the eRASS1 catalog, compared to the fluxes of all sources in the eROSITA survey [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]
Figure 6
Figure 6. Figure 6: The flux and counts distribution in the 2.0-5.0 keV band for the main eRASS1 catalog (black) and hard eRASS1 catalog (blue) compared with the flux and counts for TeV-detected HBLs (red). sult, the remaining 8 sources are considered new blazar candidates [PITH_FULL_IMA…
Figure 7
Figure 7. Figure 7: Example of the synchrotron SED of selected source THC J0819-079 (green triangles) compared to po￾tential candidate WISEA J095233.24+114520.4 that is dis￾carded by the αR-IR cut (red squares). Quasar-like sources can make it through our selection criteria based on infra…

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