Discovery of the TeV Emission from the JetInteraction Regions of SS 433 with HAWC

Chang Dong Rho; Hao Zhou; Segev BenZvi (for the HAWC Collaboration)

REVIEW 4 major objections 4 minor 12 references

Discovery of the TeV Emission from the JetInteraction Regions of SS 433 with HAWC

T0 review · 4 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Follow-up HAWC measurements with 1,039 days of data and a new energy estimator reproduce the published TeV jet-lobe fluxes of SS 433, strengthening the case that the emission is real and persistent.

desk verdict A short, honest ICRC proceeding that re-processes HAWC data with a new energy estimator and confirms the published SS 433 lobe fluxes, though it adds no new physics and its systematics are still missing. read the letter →

arxiv 1908.06429 v1 pith:YWLESLTE submitted 2019-08-18 astro-ph.HE

Chang Dong Rho , Hao Zhou , Segev BenZvi (for the HAWC Collaboration) This is my paper · ORCID

classification astro-ph.HE

keywords SS433HAWCTeVgamma-rayastronomymicroquasarjetsjetinteractionregionsW50leptonicemissionveryhighenergygammarays

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 sets out to confirm the 2018 HAWC discovery that the microquasar SS 433 emits multi-TeV gamma rays from the two jet interaction regions, e1 and w1, where the jets collide with the W 50 supernova remnant about 40 parsecs from the binary. Using 1,039 days of HAWC data and a new energy estimator that uses only well-contained on-array events, the authors simultaneously fit the two lobes and the nearby extended source MGRO J1908+06. They obtain flux normalizations at 20 TeV of $2.5^{+1.1}_{-0.8} \times 10^{-16}$ for e1 and $3.5^{+1.2}_{-0.9} \times 10^{-16}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ for w1, in good agreement with the published values. A sympathetic reader would care because an independent data set and energy reconstruction that reproduce the original signal would confirm that the TeV emission is a persistent feature rather than a statistical fluctuation or an artifact of one analysis method.

What carries the argument

The analysis is carried by a simultaneous maximum-likelihood fit of three sources: point-source models for the two SS 433 jet lobes and an electron diffusion morphology for MGRO J1908+06, the bright extended TeV source that contaminates the field. A semi-circular region of interest excludes the part of MGRO J1908+06 nearest the Galactic plane, avoiding the need to model diffuse Galactic emission. The new ingredient is the HAWC energy estimator, which uses on-array events (showers whose cores land on the main array) and is designed to be more reliable than the fractional-hit-bin method above about 10 TeV, where most photomultiplier tubes are hit. The fit uses a power-law spectrum with the index fixed at 2.0 and pivot energy 20 TeV.

What would settle it

Compute the energy-estimator systematic uncertainties for the e1 and w1 fits and check whether the flux normalizations at 20 TeV remain within the published values; a shift exceeding the statistical errors would overturn the confirmation claim.

Watch

Extended reading notes

Core claim

The central claim is that the TeV gamma-ray emission from the SS 433 jet termination regions is real and persistent, and that it remains consistent with compact, point-like sources at the X-ray-defined positions e1 and w1. The new measurement with the energy estimator yields fluxes that agree with the earlier fractional-hit-bin analysis: at 20 TeV, $dN/dE_{e1} = (2.5^{+1.1}_{-0.8}) \times 10^{-16}$ and $dN/dE_{w1} = (3.5^{+1.2}_{-0.9}) \times 10^{-16}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$. Because the lobes appear as point sources and do not show the wide angular spread expected if the gamma rays came from pion decay of spreading protons, the paper argues the measurement supports a leptonic production mechanism, though a hadronic-only model is not completely ruled out.

Load-bearing premise

The measurement stands on the assumption that the new energy estimator and the on-array-only event selection reconstruct energies and exposure with no unaccounted energy-dependent bias large enough to change the fitted fluxes.

Editorial extensions

If this is right

The e1 and w1 lobes remain detected at TeV energies with an independent event selection and energy reconstruction, so the original discovery is not tied to the fractional-hit-bin method alone.
The agreement of the flux normalizations means the spectral energy distribution of the lobes is consistent with a power law of index 2.0 around 20 TeV.
The point-source appearance of the lobes favors in-situ acceleration of electrons at the jet termination regions and disfavors gamma rays from a widely spread hadronic proton population.
With additional cumulative data, the lobe spectra can be measured in more detail, including the systematic uncertainties currently missing from the energy-estimator results.
The residual maps after subtracting the fitted sources are consistent with background, validating the multi-source modeling of the crowded SS 433 and MGRO J1908+06 field.

Reading between the lines

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

Editorial inference: If the missing systematic uncertainties on the energy estimator shift the flux normalizations by more than their statistical errors, the apparent agreement with the published values could weaken, so the confirmation is not fully closed until those systematics are reported.
Editorial inference: A direct comparison of the on-array-only and off-array event samples could reveal whether the energy-estimator selection introduces an energy-dependent exposure that partially mimics the published flux.
Editorial inference: The same simultaneous-fitting approach with an energy estimator could be applied to other compact TeV sources near the Galactic plane to test whether the point-source, leptonic interpretation for SS 433 is specific to W 50 or more general among microquasars.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Unofficial take: this is exactly what it looks like — a conference proceeding that re-analyzes the same HAWC data with a newer energy estimator and confirms the earlier Nature detection. The genuinely new piece is the energy-estimator analysis, validated on the Crab in ref [8], applied to SS 433 with 1,039 days of on-array events. The paper is transparent that it is a follow-up and that the analysis chain goes back to [4] and [8]. It does not oversell the result.

What it does well: the residual maps and significance histograms are the right checks, and they support the claim that the simultaneous fit subtracts MGRO J1908+06 and the two lobes cleanly. The e1 flux is consistent with the published value. That is real confirmation value, even if it is not an independent detection.

Soft spots, in order. The energy-estimator numbers in Table 1 have no systematic uncertainties; the text says so, but that means the quoted errors are statistical only, so the "good agreement" claim for w1 is weaker than it looks. New w1 is 3.5 vs the published 2.1 in units of 1e-16 TeV^-1 cm^-2 s^-1 — about one sigma before adding energy-scale or effective-area systematics. Second, the new dataset uses only on-array events while the published analysis also used off-array events, so the two measurements are neither event-independent nor selection-identical. And 1,017 of the 1,039 days are the same data, so this is a reprocessing confirmation, not evidence that the emission persists on a fresh time baseline. Third, the leptonic-origin remark leans on point-source morphology; that argument was already in the Nature paper, and a point-source limit alone does not sharply discriminate leptonic from hadronic. The paper itself hedges this appropriately.

Overall, the central claim holds up: the lobes are seen again in TeV gamma rays with consistent flux. The missing systematics and the selection difference should be fixed before anyone quotes these as standalone measurements.

Who this is for: someone tracking HAWC analysis methods or SS 433/W50 gets a compact confirmation, but should still cite the Nature paper for the discovery. For peer review: it is a legitimate measurement worth refereeing as a proceedings or short letter, but I would not expect a main journal to treat it as new physics.

Referee Report

4 major / 4 minor

Summary. This ICRC 2019 proceedings paper reports a follow-up HAWC measurement of the TeV gamma-ray emission from the jet interaction regions e1 and w1 of the microquasar SS 433. Using 1,039 days of HAWC data and a new energy estimator that uses only on-array events, the authors perform a simultaneous maximum-likelihood fit of the two lobes and the nearby extended source MGRO J1908+06, with power-law spectra of fixed index 2.0 and a pivot energy of 20 TeV. The fitted flux normalizations at 20 TeV are dN/dE_e1 = (2.5^{+1.1}_{-0.8}) x 10^{-16} TeV^{-1} cm^{-2} s^{-1} and dN/dE_w1 = (3.5^{+1.2}_{-0.9}) x 10^{-16} TeV^{-1} cm^{-2} s^{-1}, compared with the published values of 2.4^{+0.6+1.3}_{-0.5-1.3} and 2.1^{+0.6+1.2}_{-0.5-1.2} from Abeysekara et al. 2018. The paper argues that this agreement supports the earlier discovery and the leptonic interpretation of the emission. Residual significance maps and pixel-significance histograms are shown to demonstrate that the fitted source model leaves a residual map consistent with background.

Significance. If the new measurement is taken at face value, it provides a useful cross-check of the only known VHE gamma-ray detection from a microquasar's jet termination regions, and it supports the conclusion that the lobes are point-like at HAWC angular resolution. The paper makes good use of a new, higher-resolution energy estimator that was previously validated on the Crab Nebula, and the residual maps and significance histograms provide a clear visual check that the multi-source fit does not leave obvious unmodeled excess or oversubtraction. However, the significance of the result is limited because the bulk of the data are the same as the earlier publication, the new energy-estimator results are presented without systematic uncertainties, and the w1 flux differs from the published value by more than the wording 'good agreement' implies. These issues must be addressed before the confirmation claim can be considered quantitatively established.

major comments (4)

[Section 4, Table 1] The Table 1 caption and Section 4 state that the energy-estimator results are currently missing systematic uncertainties. Since the central claim is that the new flux normalizations are in agreement with the published values, and the published values include large systematic uncertainties (e.g., +1.3/-1.3 on e1), a comparison without any estimate of the energy-scale or effective-area systematic uncertainty is incomplete. The authors should either provide a systematic uncertainty for the energy-estimator fluxes or explicitly state which range of agreement can be claimed before systematics are included.
[Section 3, Section 6] The 'follow-up' dataset is not substantially independent of the earlier detection: 1,017 of the 1,039 days are the same data used in reference [4], with only 22 additional days. Thus the agreement between the two analyses is primarily a reprocessing of the same events with a different energy estimator and event selection, not new evidence that the emission persists on an independent dataset. The language in the abstract and Section 6 ('confirms', 'follow-up measurements') should be tempered, and the overlap of the datasets should be stated explicitly when the comparison is discussed.
[Table 1, Section 5] The agreement for the w1 lobe is weaker than stated. The new flux is 3.5^{+1.2}_{-0.9} x 10^{-16} TeV^{-1} cm^{-2} s^{-1}, while the published value is 2.1^{+0.6+1.2}_{-0.5-1.2} x 10^{-16} TeV^{-1} cm^{-2} s^{-1}. The central values differ by 1.4 x 10^{-16}, which is about 1 sigma of the new statistical uncertainty and more than 2 sigma of the published statistical uncertainty. Before the missing systematic uncertainties are added, calling this 'good agreement' is an overstatement; a quantitative compatibility statement (e.g., a chi-square or p-value) is needed.
[Section 3] The comparison between the new and published results changes two analysis ingredients at once: the energy estimator and the event selection (on-array only versus on-array plus off-array). If the on-array selection has an energy-dependent efficiency, the flux normalization at 20 TeV could shift. The authors should either apply the new energy estimator to the same event sample used in [4], or apply the old fractional-hit-bin analysis to the on-array-only sample, to isolate the effect of each change on the fitted normalizations.

minor comments (4)

[Abstract] The luminosity unit in the abstract, '10^40 erg s^-2', should be 'erg s^-1' (or erg/s).
[Author list] There is a typo in the author list line: 'F or a complete author list' should read 'For a complete author list'.
[Section 2] The phrase 'more than 99.9% originating from cosmic rays' would read better as 'more than 99.9% of which originate from cosmic rays'.
[References] The reference list entries are not in a consistent style; for example, reference [1] mixes a 2004 publication date with a pagination that includes the journal name out of order. A uniform journal formatting would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the follow-up measurement is an independent re-analysis with a separately validated energy estimator compared against a published result.

full rationale

This paper does not derive its central claim from the quantity it claims to confirm. The new flux normalizations for e1 and w1 are obtained with the HAWC energy-estimator event sample, while the comparison values come from reference [4]; the energy estimator itself is not fitted to SS 433 but was developed and validated on the Crab Nebula in reference [8]. The spectral index and pivot energy are fixed a priori (index 2.0, pivot 20 TeV), and the lobe positions are fixed to the known X-ray termination regions, so no fitted parameter is recycled as a prediction. The agreement with the earlier fractional-hit-bin analysis is an external comparison, not an input to the fit. The paper does rely on the authors' own prior work for the analysis chain, but that is a normal methodological citation rather than a load-bearing circularity: the new analysis uses a different event selection and energy estimator, and the quoted results carry statistical uncertainties only. The explicitly noted absence of systematic uncertainties and the substantial overlap of the 1,039 days with the 1,017 days used in [4] are legitimate concerns about the strength of the confirmation, but they are evidence-quality issues, not cases where the conclusion is equivalent to the premise by construction. No circular step satisfying the quoted-evidence requirement is present.

Assumptions & free parameters 4 free parameters · 3 assumptions · 0 invented entities

No new physical entities are introduced. The free parameters are the fitted flux normalizations and the fixed spectral assumptions. The axioms cover the detector framework, the source morphology model, and the energy estimator, all inherited from prior HAWC publications.

free parameters (4)

Spectral index alpha (fixed) = 2.0
The power-law index of both lobes is fixed at 2.0 in Equation (3.1), not measured. The quoted flux normalizations are conditional on this choice.
Pivot energy E_piv = 20 TeV
The differential flux is quoted at 20 TeV; a conventional choice that does not affect the detection significance but scales the normalization values.
Flux normalization A_e1 = 2.5^{+1.1}_{-0.8} x 1e-16 TeV^-1 cm^-2 s^-1 at 20 TeV
Fitted from HAWC data in the simultaneous likelihood fit; this is the measured result, not an ad hoc input.
Flux normalization A_w1 = 3.5^{+1.2}_{-0.9} x 1e-16 TeV^-1 cm^-2 s^-1 at 20 TeV
Fitted from HAWC data in the simultaneous likelihood fit; this is the measured result, not an ad hoc input.

assumptions (3)

domain assumption The HAWC maximum-likelihood fitting framework (refs [9,10]) correctly models the detector response and backgrounds.
The paper uses this framework without re-deriving it.
ad hoc to paper The MGRO J1908+06 TeV source is modeled with the electron diffusion morphology of Abeysekara et al. 2017 (ref [11]), and the semi-circular region of interest excludes the upper half of this source without biasing the lobe fits.
This modeling choice is introduced in Section 3 to avoid modeling Galactic diffuse emission; its validity is not demonstrated in this paper.
domain assumption The new HAWC energy estimator (ref [8]) correctly reconstructs photon energies for on-array events.
The paper relies on this estimator but does not validate it here; it references the Crab Nebula paper [8] for performance.

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Cite this review

Pith. "Pith review of Discovery of the TeV Emission from the JetInteraction Regions of SS 433 with HAWC." pith.science (2026). https://pith.science/paper/YWLESLTE

@misc{pith2026190806429,
  author       = {Pith},
  title        = {Pith review of: Discovery of the TeV Emission from the JetInteraction Regions of SS 433 with HAWC},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YWLESLTE}},
  note         = {Machine review of arXiv:1908.06429}
}

abstract

The High Altitude Water Cherenkov (HAWC) observatory recently published the discovery of SS 433 as a TeV source, reporting the observation of multi-TeV gamma-ray emission from the jet interaction regions e1 and w1, suggesting in-situ particle acceleration. This showed the first direct evidence of acceleration in jets at energies greater than a few TeV. SS 433 was the first microquasar to be discovered and is still considered special in that the accretion is supercritical and the luminosity of the system is very high ($\sim10^{40}$ erg s$^{-2}$). The lobes of the supernova remnant W 50 in which the jets terminate, about 40 parsecs from the central binary, are expected to accelerate charged particles, and indeed radio and X-ray emission consistent with electron synchrotron emission in a magnetic field have been observed. SS 433 has also been a strong candidate for hadronic acceleration due to spectroscopic evidence of ionized nuclei in the inner jets. However, multiwavelength fits including the HAWC measurements favor the leptonic production of the observed gamma rays. Here, we present new follow-up measurements of the jet interaction regions of SS 433 using the most recent data from HAWC.

Figures

Figures reproduced from arXiv: 1908.06429 by the authors.

**Figure 1.** Top: The original significance map of the SS 433 region. Bottom Left: The residual map after subtracting the fitted MGRO J1908+06 model. Bottom Right: The residual map after subtracting the fitted MGRO J1908+06 and the SS 433 lobes model. Although the plots here show successive subtraction of source models, the joint likelihood fit is actually performed simultaneously. Plots are only presented this way to better ill… view at source ↗

**Figure 2.** Distribution of pixel significance in the semi-circular RoI. The best fit Gaussian is in green dot-dashed curve. Top: Corresponds to the pixels in the semi-circular RoI of [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

12 extracted references · 8 canonical work pages

[8]

Abeysekara, A. U. et al. (HAWC Collaboration) (2019) Measurement of the Crab Nebula at the Highest Energies with HAWC, arXiv:1905.12518

arXiv 2019
[4]

Abeysekara, A. U. et al. (HAWC Collaboration) (2018) Very high energy particle acceleration powered by the jets of the microquasar SS 433. Nature, 562:82-85

work page 2018
[1]

(2006) The jets and and supercritical accretion disk in ss433, Comments Astrophys

Fabrika, S. (2006) The jets and and supercritical accretion disk in ss433, Comments Astrophys. Space Phys., 12:1-153, 2004

work page 2006
[2]

and Ogelman, H

Saﬁ-Harb, S. and Ogelman, H. (1997) Rosat and asca observations of w50 associated with the peculiar source ss 433, Astrophys. J., 483(2):868-881

work page 1997
[3]

(1984) Observations of SS 433

Margon, B. (1984) Observations of SS 433. Ann. Rev. Astron. Astrophys., 22:507-536

work page 1984
[5]

Abeysekara, A. U. et al. (HAWC Collaboration) (2017) Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory, arXiv:1701.01778. 6 HA WC SS 433 F ollow-Up Hao Zhou

arXiv 2017
[6]

Smith, A. J. et al. (HAWC Collaboration) (2015) HAWC: Design, Operation, Reconstruction, and Analysis, in Proc. 34th ICRC, arXiv:1508.05826v2

arXiv 2015
[7]

BenZvi, S. Y . (HAWC Collaboration) (2015) Looking for Gamma-ray Emission from TeV Binary Candidates with HAWC, in Proc. 34th ICRC, arXiv:1508.03589

work page Pith review arXiv 2015

Show all 12 references

[9]

Younk, P. W. et al. (HAWC Collaboration) (2016) A high-level analysis framework for HAWC, Proc. Sci. (34th Int. Cosmic Ray Conf.) ICRC2015, https://doi.org/10.22323/1.236.0948

2016 doi
[10]

Vianello, G. et al. (HAWC Collaboration) (2016) The multi-mission maximum likelihood framework, Proc. Sci. (34th Int. Cosmic Ray Conf.) ICRC2015, https://doi.org/10.22323/1.236.1042

2016 doi
[11]

Abeysekara, A. U. et al. (HAWC Collaboration) (2017) Extended gamma-ray sources around pulsars constrain the origin of the positron ﬂux at Earth, Science, 358(6365):911-914

2017
[12]

(1996) ROSAT observations of the W 50/SS 433 system

Brinkmann, W., Aschenbach, B., and Kawai, N. (1996) ROSAT observations of the W 50/SS 433 system. Astron. Astrophys., 323:306-316. 7

1996

Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [8]

Abeysekara, A. U. et al. (HAWC Collaboration) (2019) Measurement of the Crab Nebula at the Highest Energies with HAWC, arXiv:1905.12518

arXiv 2019

[2] [4]

Abeysekara, A. U. et al. (HAWC Collaboration) (2018) Very high energy particle acceleration powered by the jets of the microquasar SS 433. Nature, 562:82-85

work page 2018

[3] [1]

(2006) The jets and and supercritical accretion disk in ss433, Comments Astrophys

Fabrika, S. (2006) The jets and and supercritical accretion disk in ss433, Comments Astrophys. Space Phys., 12:1-153, 2004

work page 2006

[4] [2]

and Ogelman, H

Saﬁ-Harb, S. and Ogelman, H. (1997) Rosat and asca observations of w50 associated with the peculiar source ss 433, Astrophys. J., 483(2):868-881

work page 1997

[5] [3]

(1984) Observations of SS 433

Margon, B. (1984) Observations of SS 433. Ann. Rev. Astron. Astrophys., 22:507-536

work page 1984

[6] [5]

Abeysekara, A. U. et al. (HAWC Collaboration) (2017) Observation of the Crab Nebula with the HAWC Gamma-Ray Observatory, arXiv:1701.01778. 6 HA WC SS 433 F ollow-Up Hao Zhou

arXiv 2017

[7] [6]

Smith, A. J. et al. (HAWC Collaboration) (2015) HAWC: Design, Operation, Reconstruction, and Analysis, in Proc. 34th ICRC, arXiv:1508.05826v2

arXiv 2015

[8] [7]

BenZvi, S. Y . (HAWC Collaboration) (2015) Looking for Gamma-ray Emission from TeV Binary Candidates with HAWC, in Proc. 34th ICRC, arXiv:1508.03589

work page Pith review arXiv 2015