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The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs

Aafia Zainab, Philipp Thalhammer, Nico Zalot, Artur Avakyan, Jakob Stierhof, Ekaterina Sokolova-Lapa, Victor Doroshenko, Victoria Grinberg, Peter Kretschmar, Galina Lipunova, Nazma Islam, Matthias R. Schreiber, Christian Kirsch, Steven Hämmerich, Philipp Weber, Ralf Ballhausen, Alicia Rouco Escorial, Joel Coley, Richard Rothschild, Katja Pottschmidt, Joern Wilms

arXiv:2608.13259v1astro-ph.HEastro-ph.GA

TL;DR

The low-luminosity behavior of BeXRBs has lacked systematic, unbiased evidence because existing surveys poorly constrained faint systems. Using four eROSITA surveys to extend and subtype-resolve the Galactic HMXB luminosity function, the paper finds that BeXRBs are commonly detected at low luminosities, supporting accretion outside outburst as the norm.

  • Problem

    The low-luminosity HMXB distribution and the universality of sustained low-level BeXRB accretion remain poorly constrained by existing surveys and samples.

  • Method

    The study analyzes four eROSITA surveys of known Galactic HMXBs, constructs subtype-resolved luminosity functions, models variability, and compares them with RXTE/ASM and MAXI.

  • Results

    The HMXB luminosity function extends by ≳3 orders of magnitude, with SgXBs at higher luminosities and BeXRBs contributing substantially below 10^35 erg s−1.

  • Takeaways & Limitations

    At least 60% of BeXRBs are detected in a single survey and at least 80% across four surveys at 10^33–10^35 erg s−1, supporting accretion outside outburst as common.

  • Takeaways & Limitations

    The constraint that fewer than 20% of systems may be consistently hindered by the propeller effect is based only on the Western hemisphere.

Abstract

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We present a first look at the Galactic population of heretofore known HMXBs as observed by SRG/eROSITA during its first four surveys. eROSITA's sensitivity of $\sim10^{-13}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, translating to $10^{32}$-$10^{34},\mathrm{erg}\,\mathrm{s}^{-1}$ in luminosity for most known HMXBs in the Milky Way, has thus far never been reached by any wide-area survey instrument. We present the extended log N-log L distribution of known HMXBs reaching down to $10^{32}\,\mathrm{erg}\,\mathrm{s}^{-1}$ using eROSITA, and show the large scatter that can be induced by source intrinsic variability. We present sub-type resolved luminosity distributions, showing that the Supergiant X-ray binaries (SgXBs) and Be X-ray binaries (BeXRBs) occupy different parts of the overall distribution, and reanalyse RXTE/ASM data and MAXI for comparison to eROSITA. The luminosity regime uncovered by eROSITA allows a systematic study of the "transient" BeXRBs, which are typically below the detection threshold of monitors outside of outburst, and whose low luminosity behavior has been a longstanding question. Signatures of stable accretion at low luminosities have been observed with pointed instruments for a fraction of the overall sample, so far. With the eROSITA results, we posit that accretion outside of outburst is likely the norm, since a vast majority (> 80%) of BeXRBs are detected at luminosities at least an order of magnitude higher than expected for the most X-ray luminous Be stars. We discuss the observed luminosity in the context of cold disk accretion and the "propeller" mechanism. We highlight a small subpopulation of "isolated" Be-stars that reach luminosities comparable to the least luminous BeXRBs, hinting at the presence of compact object companions.

1. Introduction

HMXBs comprise predominantly persistent SgXBs and predominantly transient BeXRBs, but their low-luminosity distribution remains poorly constrained. eROSITA enables a systematic study of faint known HMXBs, especially BeXRB accretion outside outbursts.

  • HMXB subclasses: HMXBs are divided into predominantly persistent SgXBs with OB-type supergiant donors and predominantly transient BeXRBs with main-sequence Be-star donors and decretion disks.SgXBs are associated with supergiant companions, whereas BeXRB optical emission lines arise from circumstellar decretion disks.
  • Low-luminosity gap: X-ray luminosity functions have largely been constrained only down to ∼10^35 erg s^-1 because earlier instruments lacked sufficient sensitivity at lower luminosities.The lower-luminosity end of the HMXB distribution therefore remains poorly characterized.
  • Low-luminosity accretion: Reduced accretion scenarios are especially important for transient BeXRBs and SFXTs, with BeXRB behavior outside outbursts particularly relevant to the low-luminosity XLF.Typical supergiant systems can remain observable at mostly stable luminosity for most of their orbit.
  • Low-luminosity accretion: ∼20% of the known Milky Way BeXRB sample shows sustained accretion at low mass accretion rates and luminosities, but its universality remains untested systematically.The existing evidence comes from individual BeXRBs rather than an unbiased systematic sample.
  • eROSITA approach: eROSITA’s all-sky capability and improved sensitivity are suited to detecting known HMXBs at much lower luminosities, including sources with rare outbursts or flares.The study focuses on the hitherto known HMXB sample to characterize faint systems and support identification of additional faint HMXBs.

2. Sample, Cross-matching, and Data

The study analyzes Milky Way HMXBs in eROSITA’s Western Galactic hemisphere using an updated, subtype-classified catalog and repeated eRASS cross-matching. Sources were matched geometrically, filtered by detection likelihood, and processed separately as detections or non-detections for flux measurements and upper limits.

  • Sample definition: The analysis is restricted to the Western Galactic hemisphere, defined west of Sgr A∗ at lII ≃0°.The authors’ access through the eROSITA_DE consortium covered only this region.
  • Sample definition: 72 HMXBs remained in the revised catalog from 83 sources after Western-hemisphere filtering and literature-based subtype reclassification.The catalog was based on XRBcats and incorporated updates from Fortin et al. (2023).
  • Cross-matching: Sources were cross-matched to eRASS catalogs within ∼15″ and retained only when DET_LIKE ≥ 10.The same procedure was repeated for subsequent eRASS catalogs; sources failing these criteria were treated as non-detections for flux upper limits.
  • Data processing: Detected and non-detected sources were separated during data reduction, with flux products extracted using eSASS version 211214 and processing version 020.Detected sources were processed with the eSASS CATPREP option using source coordinates, count rates, background rates, and DETUIDs.

3. Flux and Luminosity Distributions

eROSITA extends Galactic HMXB luminosity distributions to lower fluxes and luminosities than previous wide-area monitors, revealing subtype-dependent distributions and substantial variability. BeXRBs dominate the lower-luminosity range, whereas SgXBs dominate the high-luminosity end.

  • Flux sensitivity and completeness: eROSITA is effectively complete above 10^-13 erg cm^-2 s^-1, with completeness down to 10^33 erg s^-1 within ∼10 kpc.Non-detections above 10^-13 erg cm^-2 s^-1 are therefore more likely source-intrinsic than sensitivity-limited.
  • Subtype luminosity distributions: The higher luminosity range is dominated by classical SgXBs, while BeXRBs dominate the lower luminosity range.No BeXRBs were found above 10^37 erg s^-1, the typical luminosity of Type II outbursts.
  • Variability: The four eRASS luminosity-function fits vary substantially, especially Γ2 and Lbreak, depending on the number of sources flaring or in brighter states.The authors estimated overall variation by randomly assigning each source a luminosity from one of its four eRASS detections.
  • Comparison with monitoring instruments: RXTE/ASM and MAXI agree with eROSITA above their sensitivity thresholds but do not extend significantly below 10^35 erg s^-1.Both monitors observe outbursts above 10^36 erg s^-1, while MAXI reaches only a few 10^35 erg s^-1.
  • Comparison with monitoring instruments: Previous monitor-based luminosity functions were dominated by SgXBs and biased toward persistent systems because their sensitivity primarily probed luminosities ≳10^36 erg s^-1.The lower-luminosity BeXRB population was consequently underrepresented.

4. Comparison to Be stars

eROSITA-detected BeXRBs are generally at least an order of magnitude more luminous than volume-limited isolated Be stars, supporting accretion as their dominant low-luminosity emission source. However, overlap at 10^32–10^33 erg s−1 requires spectral and hard-band diagnostics to distinguish luminous Be stars from faint BeXRBs.

  • Luminosity comparison: Most eROSITA-detected BeXRBs show significant excess luminosities relative to those expected from their Be-star donors.Undetected BeXRB systems also have upper limits above the expected donor luminosity threshold.
  • Luminosity comparison: At least an order of magnitude: BeXRBs are detected at higher luminosity than the volume-limited isolated Be-star sample.The comparison uses eROSITA log N-log L distributions, including Be stars within 500 pc as a less biased reference sample.
  • Luminosity overlap: 10^32–10^33 erg s−1: Be stars and BeXRBs significantly overlap in overall 0.2–10.0 keV luminosity.This overlap makes distinguishing the most luminous Be stars from low-luminosity BeXRBs important.
  • Spectral distinction: Above 2 keV, Be stars have significantly less flux than BeXRBs, providing a hard-band discriminator between the populations.Their spectra can also differ: isolated Be stars are purely thermal and drop off after 1 keV, whereas BeXRBs are harder and prominent above 1 keV.
  • Candidate companions: A handful of apparently isolated Be stars are as luminous as the least luminous BeXRBs, especially in the hard band.These sources may represent luminous Be-star emitters or systems with compact-object companions, motivating further hard-X-ray observations.

5. What are BeXRBs doing at low luminosities?

eROSITA finds that most BeXRBs remain detectable outside outburst at luminosities well above those expected from Be stars, while distinct spin-period populations show differing low-luminosity behavior. These detections support ongoing accretion in many systems, although propeller, leakage, and cold-disk scenarios remain difficult to distinguish.

  • Observed luminosity distribution: 80% of known BeXRBs are detected above expected Be-star luminosities when combining eRASS1:4 data and excluding Type I outbursts.In a single eRASS, ≳60% are detected above this expectation, typically at ≳10^33 erg s−1 in the 0.2–10 keV band.
  • Observed luminosity distribution: The largest population has spin periods ≳100 s and luminosities of ∼10^33–10^35 erg s−1, consistent with stable accretion outside outburst.These sources are detected a vast majority of the time at 10^33–10^34 erg s−1, with some confirmed hard X-ray emission.
  • Observed luminosity distribution: Short-period systems are generally observed at ≳10^36 erg s−1 during outburst, then decline to non-detections or lower-luminosity detections.Three systems with orbital periods of 20–50 d show repeated luminous states, while their limited low-luminosity coverage does not rule out continued accretion.
  • Observed luminosity distribution: A predominantly undetected group has stringent upper limits, with the lowest reaching ≲10^32 erg s−1; only six sources are absent from every eRASS scan.LS 992 and AX J1700.2−4220 cross the detection threshold after combining eRASS data, with AX J1700.2−4220 near ∼10^32 erg s−1.
  • Caveats and comparison: Luminosity interpretations carry substantial uncertainty because estimates may be off by up to a factor 10, while absorption and magnetic fields are often poorly constrained.The same absorption issue limits interpretation of eROSITA non-detections for SgXBs, whose wind-fed systems are often intrinsically absorbed.
  • Physical interpretation: Short-period detections near the luminosity gap could reflect cold-disk accretion, magnetospheric leakage during the propeller regime, or an intermediate transition plateau.Some systems reach the lowest propeller threshold, ∼10^34 erg s−1 for a magnetic field of ∼10^12 G, but the available eROSITA statistics require hard X-ray follow-up.

6. Take-aways for the XLF from eROSITA

The eROSITA luminosity function requires a break at low luminosities, while variability and HMXB subclass differences are essential for interpreting Galactic XLFs and comparisons with external galaxies. Completing the XLF will require candidate characterization, Galactic population modelling, and complementary hard-X-ray and survey data.

  • XLF shape: The eROSITA data clearly require a break in the HMXB XLF, contrary to the suggestion that no cutoff was needed down to 10^34 erg s−1.This conclusion follows from the expanded luminosity distributions discussed in the section.
  • Future work: A more complete XLF requires accurate characterization of eROSITA candidate HMXBs, Galactic HMXB population-density modelling, and combination with hard-X-ray information.The authors also propose sensitive hard-X-ray surveys or unbiased survey-like follow-up, with NuSTAR and Einstein Probe identified as useful for expanding the sample.
  • Variability: Variability from outbursts and flares changes the XLF shape regardless of energy range and must be included in Milky Way luminosity-function studies.Accounting for variability is especially important when comparing Galactic inferences with external galaxies observed through only a few snapshots.
  • Subclass differences: Splitting HMXBs into BeXRBs, SgXBs, and SFXTs helps identify which sources are likely to appear in random snapshots of distant galaxies.Only one or two BeXRBs were consistently detected during the decade of RXTE/ASM observations because they spend most of their lifetimes below the detection threshold.
  • Binary evolution: The low-luminosity end, though largely inaccessible in other galaxies, contains important information for binary-evolution scenarios.The authors place the comparison with isolated Be stars in the context of binary population studies.

7. Are some Be stars harboring quiet BeXRBs?

The brightest Be stars with anomalously hard and luminous X-ray emission may harbor binary companions, potentially including quiet BeXRBs. This possibility merits renewed consideration because BeXRBs can remain below outburst levels for several years.

  • Reconsidering bright Be stars as BeXRB candidates: Some Be stars emit X-rays comparable to the least luminous BeXRBs, although known BeXRBs generally show excess luminosity over isolated Be stars.The excess emission in known BeXRBs is taken as evidence for ongoing accretion, while the origin of emission from the brightest Be stars is reconsidered.
  • Reconsidering bright Be stars as BeXRB candidates: Binary interaction is the most favored explanation for anomalously hard X-ray luminosities in some of the brightest Be stars.This interpretation is supported by eROSITA survey results and theoretical arguments for binarity.
  • Reconsidering bright Be stars as BeXRB candidates: Bright, hard, luminous Be stars without obvious γ Cas signatures should be revisited as BeXRB candidates because many BeXRBs can remain quiescent for several years.Their previously low luminosity relative to neutron-star binaries had argued against classifying them as potential HMXBs.

8. Summary and Conclusion · Appendix A: HMXB Catalog and comparison to previous missions

eROSITA extends the Galactic HMXB luminosity function to much lower luminosities, separates SgXBs and BeXRBs, and reveals widespread low-luminosity BeXRB detections consistent with accretion outside outburst. The updated catalog retains positional, timing, and source-classification information while deriving companion-object labels from XRBcats descriptors.

  • 8. Summary and Conclusion: ≳3 orders of magnitude: eROSITA extends the HMXB XLF, with completeness to ∼10^33 erg s−1 at 10 kpc for a ∼10−13 erg s−1cm−2 sensitivity limit.The survey reaches the low-luminosity regime previously inaccessible to wide-area monitoring.
  • 8. Summary and Conclusion: SgXBs dominate higher luminosities, whereas BeXRBs significantly contribute to the low-luminosity end of the HMXB XLF.The subclass-resolved distribution shows a clear luminosity separation between predominantly persistent SgXBs and predominantly transient BeXRBs.
  • 8. Summary and Conclusion: A broken power law is required for the eROSITA luminosity distribution, unlike RXTE/ASM samples above 10^36 erg s−1 where no break is required.The result is consistent with earlier broken-power-law proposals by Voss & Ajello (2010) and Lutovinov et al. (2013).
  • 8. Summary and Conclusion: Two BeXRB populations emerge: systems with stringent upper limits below the stable-accretion luminosity and systems consistently detected below outburst luminosities.The available data do not yet distinguish among the relevant low-luminosity accretion models, and any propeller effect appears marginal for the known sample.
  • Appendix A: HMXB Catalog and comparison to previous missions: The updated HMXB catalog retains SIMBAD identifiers, XRBcats right ascension and declination, pulse and orbital periods, and X-ray-type descriptors such as companion type and cyclotron lines.These columns provide source identity, position, timing, and spectral-feature information for the catalog used in the analysis.
  • Appendix A: HMXB Catalog and comparison to previous missions: NS and BH labels are assigned directly when present, while XP, CL, and QPO descriptors also trigger NS classification because they indicate accreting neutron stars.The CO and TYPE columns are derived by parsing the Xray_Type field of XRBcats.

A.1. Reclassification · Appendix B: Spectral fitting

The reclassification removes several sources with weak or disputed HMXB identifications, while retaining some debated Be-star systems and classifying SFXTs from the literature. Appendix B documents eROSITA spectral fitting and pile-up corrections affecting selected bright sources.

  • A.1. Reclassification: CCDM J07474−5320A and 1H 0749−600 are excluded because their HMXB nature is considered tenuous.CCDM J07474−5320A has a very high parallax and was initially classified as a Be+WD system, while 1H 0749−600 was labeled tentative.
  • A.1. Reclassification: HD 141926 and HD 249179 are removed because of their alternative classifications or prior exclusion from the HMXB sample.HD 141926 is identified as an Herbig Ae/Be star and a candidate white-dwarf system, whereas HD 249179 was discarded by Fortin et al. (2022).
  • A.1. Reclassification: Table A.1 records HMXBs whose subclass or ancillary information changed relative to XRBcats.Sources marked with an asterisk were listed as candidates by Neumann et al. (2023).
  • A.1. Reclassification: HD 110432, HD 119682, µ2 Cru, and HR 4804 are retained despite debated or possible companion classifications.The sources have been discussed as γ Cas systems, included among isolated Be stars, proposed to contain an unknown companion, or searched as Be+sdOB candidates.
  • A.1. Reclassification: Several sources are classified as SFXTs using literature searches because the input catalogue lacks HMXB subtype information beyond the optical companion type.The resulting SFXT classifications are listed in Table A.1 and draw on Bozzo et al. (2015 and references therein).
  • Appendix B: Spectral fitting: eRASS1–4 pile-up affected a handful of sources, mainly bright SgXBs and BeXRBs observed during Type I outbursts.Affected SgXBs include Cen X-3, Vela X-1, Cir X-1, 4U 1538−52, and 4U 1700−377; affected BeXRBs include A 0538+263 and GRO J1008−57.
  • Appendix B: Spectral fitting: Figure B.2 compares the eRASS1 flux distribution before and after pile-up correction.The comparison addresses the subset of eRASS1 sources affected by pile-up.
  • Appendix B: Spectral fitting: An example HMXB spectral fit uses eROSITA data from an eRASS1 snapshot of 1A 0535+263.Figure B.1 shows the spectrum fitted with absorbed power-law and absorbed black-body models, alongside their residuals.

Appendix C: Fits to the eROSITA log N-log L · Appendix D: RXTE/ASM and MAXI luminosity distributions

The eROSITA HMXB and subclass log N-log L distributions require broken power-law fits, although intrinsic variability and subclass-specific structure limit parameter constraints. Comparisons with RXTE/ASM and MAXI verify the bright end while exposing limitations of earlier instruments.

  • Appendix C: Fits to the eROSITA log N-log L: Broken power-law models are required for the eROSITA HMXB sample and its main subclasses, despite deviations caused by intrinsic variability.Intrinsic variability can prevent the cutoff from being constrained, as observed for eRASS1.
  • Appendix C: Fits to the eROSITA log N-log L: BeXRB break luminosities have substantially larger uncertainties, and a single power law is only marginally inferior despite unmodeled distribution kinks.The passage notes that BeXRBs still benefit from a broken power-law model.
  • Appendix C: Fits to the eROSITA log N-log L: BeXRBs and SgXBs show different fitted Γ2 distributions, with an additional high-Γ2 peak possibly produced by particularly bright outbursts.The distributions come from fits to 1000 simulated distributions.
  • Appendix C: Fits to the eROSITA log N-log L: 6–7 BeXRBs peak in the fitted norm above 10^34 erg s^-1, while the BeXRB and SgXB subclasses have different norm distributions.The norm indicates the number of sources detected above 10^34 erg s^-1.
  • Appendix D: RXTE/ASM and MAXI luminosity distributions: The RXTE/ASM and MAXI comparisons verify eROSITA detections at the high-luminosity end and illustrate the limitations of previous instruments.The appendices describe reproducing RXTE/ASM log N-log L distributions and computing MAXI luminosity distributions.
  • Appendix D: RXTE/ASM and MAXI luminosity distributions: Appendix D details the reproduction of RXTE/ASM log N-log L distributions following Grimm et al. (2002) and the computation of MAXI luminosity distributions.These procedures support comparison with the eROSITA luminosity distributions.

D.1. RXTE/ASM

The RXTE/ASM comparison reconstructs a bright-source log N–log S distribution from matched HMXB lightcurves, using a 5 mCrab cutoff and Crab-based count-rate conversion. This cutoff enables detector-sensitivity control but biases the inferred flux distribution, especially at low count rates and during outbursts.

  • Source selection: 5 mCrab corresponds to 0.37 cts s−1 in the RXTE/ASM selection.The source cutoff follows Grimm et al.'s bright-source criterion.
  • Source selection: The analysis matched HMXB catalogs to RXTE/ASM lightcurves and retained sources reaching ≥ 0.37 cts s−1 before 2000 April 27.The matching radius was 15′′, and average rates were computed over the quoted observation period.
  • Flux conversion: The study converted RXTE/ASM count rates to flux with the Crab unit and multiplied the Western-hemisphere distribution by 2 for whole-sky projection.For RXTE/ASM, 1 Crab corresponds to ∼73.6 cts s−1.
  • Systematic effects: The 5 mCrab brightness cutoff systematically overestimates fluxes, while averaging over outbursts produces an additional overestimate.Removing the cutoff instead overestimates fluxes at very low count rates, motivating an uncertainty-based limit.

D.2. MAXI

The MAXI analysis used one-day binned lightcurves, cumulative 2–10 keV flux normalized to the Crab, and 500 random flux draws per source.

  • D.2. MAXI: MAXI lightcurves were analyzed in one-day bins through MJD61213 (2026 June 22nd), using the team-maintained data.The analysis followed the same cross-matching methods described above.
  • D.2. MAXI: Fluxes were measured in 2–4 keV, 4–10 keV, and 10–20 keV bands, with cumulative 2–10 keV flux normalized to the Crab.For each source, 500 fluxes were randomly drawn from its lightcurve.

D.3. Subclass resolved distributions · Appendix E: BeXRBs as seen by eROSITA

Subclass-resolved monitor distributions show that SgXBs occupy higher luminosities than BeXRBs, while eROSITA data support systematic low-luminosity studies of BeXRBs and related Be-star systems. Appendix E consolidates Western-hemisphere BeXRB properties and hard X-ray detections outside outburst.

  • D.3. Subclass resolved distributions: RXTE/ASM and MAXI subclass distributions are shown with distribution means and 3σ contours, using randomized iterations for each subclass.The distributions are split into SgXBs and BeXRBs for both monitors.
  • D.3. Subclass resolved distributions: MAXI distinguishes the subclasses, with SgXBs predominantly detected above a few 1035 erg s−1.RXTE/ASM lacks complete coverage below 1036 erg s−1, preventing persistent SgXB detection at all times.
  • D.3. Subclass resolved distributions: MAXI detects ∼5–10 BeXRBs on average across the full sky, compared with 2–3 detectable by RXTE/ASM at any time.MAXI benefits from improved coverage, sensitivity, and a more updated source catalog.
  • D.3. Subclass resolved distributions: The relative dearth of BeXRBs in RXTE/ASM distributions reflects their rare detection, typically during outburst.The subclass distinction is more defined for MAXI than for RXTE/ASM.
  • Appendix E: BeXRBs as seen by eROSITA: eROSITA maps fluxes and corresponding luminosities for BeXRBs, isolated Be stars, and γ Cas systems.The figure also marks suspected BeXRB candidates among especially bright Be stars and depicts Be+sdOB systems separately.
  • Appendix E: BeXRBs as seen by eROSITA: Table E.1 and Table E.2 tabulate properties of BeXRBs in the Western Galactic hemisphere using information gathered from the eROSITA survey.Table E.2 includes hard X-ray detections outside outburst to consolidate ongoing and support future low-luminosity BeXRB studies.

Appendix F: Final Catalog

The paper provides the final catalog as auxiliary material, with representative eRASS1 fields and the same format extended through eRASS4. Catalog entries include fitted-parameter confidence intervals, distance fallbacks, and special handling for upper limits.

  • Final catalog: The final catalog is supplied as auxiliary material, with Table F.1 presenting a representative set of fields using eRASS1 as an example.The catalog format is likewise used for eRASS2–4.
  • Final catalog: Subsequent survey columns use eRASS2–4 suffixes, while fitted parameters receive lower and upper 90% confidence-interval columns.For example, Gamma_e1_lo and Gamma_e1_hi record photon-index uncertainties.
  • Final catalog: For upper-limit measurements, only flux columns are populated and uncertainty columns remain empty.This distinguishes limits from fitted detections in the catalog representation.
  • Final catalog: When no reliable Bailer-Jones et al. (2021) distance exists, the catalog uses the distance from XRBcat.Each fitted parameter still has two additional uncertainty columns under the same eRASS2–4 format.
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