Fermi-LAT View on Three Ultra-high-energy 1LHAASO Sources in the $52^{\circ}<l<55^{\circ}$ Region

Chuyuan Yang; Linjie Liu; Pierrick Martin; Xian Hou

arxiv: 2605.28081 · v2 · pith:5EBTXAEEnew · submitted 2026-05-27 · 🌌 astro-ph.HE

Fermi-LAT View on Three Ultra-high-energy 1LHAASO Sources in the 52^(circ)<l<55^(circ) Region

Linjie Liu , Xian Hou , Pierrick Martin , Chuyuan Yang This is my paper

Pith reviewed 2026-06-29 11:13 UTC · model grok-4.3

classification 🌌 astro-ph.HE

keywords Fermi-LATLHAASOultra-high-energy gamma rayspulsarspulsar wind nebulaesupernova remnantsGalactic diffuse emissiongamma-ray sources

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The pith

Fermi-LAT data split the region into five GeV sources whose spectra and positions do not match the three LHAASO ultra-high-energy sources.

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

The paper analyzes more than 17 years of Fermi-LAT observations to examine gamma-ray emission in a crowded Galactic plane patch that includes three ultra-high-energy sources reported by LHAASO. It shows the GeV emission can be separated into three point-like sources and two extended sources, with identifications for some as a known pulsar and a pulsar wind nebula or supernova remnant. Spectral fits allow leptonic or hybrid particle scenarios for several sources, yet position and spectrum differences prevent firm links to any of the three LHAASO sources. The work therefore leaves the origin of the ultra-high-energy emission unclear and highlights the need for refined background modeling.

Core claim

More than 17 years of Fermi-LAT data resolve the GeV emission in the 52°<l<55° region into three pointlike sources (J1925+1729P, J1930+1851P, J1932+1916P) and two extended sources (J1929+1732E, J1930+1826E). J1932+1916P is identified as PSR J1932+1916 and J1930+1851P coincides with PWN/SNR G54.1+0.3, while the GeV-TeV spectra of the sources are consistent with leptonic or hybrid interpretations in some cases; however, spectral and morphological mismatches mean none of the five GeV sources can be clearly associated with the three 1LHAASO UHE sources.

What carries the argument

Spatial and spectral fitting of Fermi-LAT photon data to isolate point-like and extended sources from each other and from the modeled Galactic diffuse background.

If this is right

The three LHAASO ultra-high-energy sources likely have origins distinct from the identified GeV sources or involve particle populations visible only at higher energies.
Deeper HAWC and LHAASO observations combined with CTAO data at higher angular resolution can test possible associations.
A dedicated study of the Galactic diffuse emission in this region is required to confirm or revise the source catalog.
Timing analysis of J1925+1729P may reveal pulsations and establish it as a new gamma-ray pulsar.

Where Pith is reading between the lines

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

If the mismatches hold under improved modeling, the ultra-high-energy emission may trace a distinct acceleration process not coupled to the lower-energy sources.
The region could contain multiple overlapping particle accelerators whose emissions blend at current resolutions.
Refined diffuse models might uncover faint bridges of emission connecting the GeV and TeV components.

Load-bearing premise

The current model of Galactic diffuse emission is accurate enough to separate the five GeV sources reliably from one another and from the background.

What would settle it

A revised diffuse emission model that produces large changes in the residual maps after the five sources are subtracted would show the reported source separation is not robust.

Figures

Figures reproduced from arXiv: 2605.28081 by Chuyuan Yang, Linjie Liu, Pierrick Martin, Xian Hou.

**Figure 1.** Figure 1: Top left: log-scaled 3. ◦ 5 × 3. ◦ 5 smoothed counts map covering the target region in the 1−500 GeV energy range, with a spatial bin size of 0. ◦ 05, and smoothed using a 0. ◦ 15 Gaussian kernel. Overlaid are the 4FGL-DR4 sources and two new sources found in the baseline model (white plus symbol), along with the three 1LHAASO source positions (blue cross) and the corresponding 39% containment radii measur… view at source ↗

**Figure 2.** Figure 2: SEDs of the emission for the five detected sources in our target region. Red lines and cyan shaded regions represent the best-fit models with 1σ statistical uncertainties, while blue data points indicate the energy flux in each energy bin, with 95% confidence upper limits for bins with TS < 4 shown by arrows. For J1925+1729P, the PLEC4 model (black dashed line) is also shown for comparison [PITH_FULL_IMAG… view at source ↗

**Figure 3.** Figure 3: Broadband SED modeling of the three target sources. Top left: leptonic model for J1930+1851P jointly fitted with HAWC J1930, assuming an electron distribution with an ECPL spectrum. Top right: hadronic model for J1930+1851P jointly fitted with HAWC J1930, assuming a proton distribution with an ECPL spectrum. Bottom left: hadronic model for J1930+1826E, assuming a proton distribution with an ECPL spectrum. … view at source ↗

read the original abstract

Using more than 17 yr of Fermi-LAT data, we performed a detailed investigation of the complex $52^{\circ}<l<55^{\circ}$ region, which encompasses the three ultra-high-energy sources 1LHAASO J1928+1746u, 1LHAASO J1928+1813u, and 1LHAASO J1929+1846u. This region hosts multiple SNRs, pulsars, GeV and TeV sources. Our analysis resolves the GeV emission into three pointlike sources (J1925+1729P, J1930+1851P, and J1932+1916P) and two extended sources (J1929+1732E and J1930+1826E), and improves significantly on the description based on the 4FGL-DR4 catalog. Source J1932+1916P is identified as the known gamma-ray pulsar PSR J1932+1916, while J1925+1729P may be a new gamma-ray pulsar candidate distinct from the known gamma-ray pulsar PSR J1925+1720. This warrants future investigation and a search for pulsations. Source J1930+1851P coincides with the TeV source PWN/SNR G54.1+0.3 and its GeV-TeV spectrum is consistent with both leptonic and hadronic interpretations, although a leptonic origin in relation to the known PWN is more likely. The GeV-TeV spectrum of J1929+1732E is consistent with a hybrid lepto-hadronic scenario in which the TeV emission traces the PWN powered by the pulsar PSR J1928+1746, while the GeV emission may result from interactions between particles escaped from the parent SNR and illuminating the gas environment. Similarly, J1930+1826E is likely connected to PWN/SNR G54.1+0.3 under a hadronic scenario involving escaped particles in their early propagation stage. Owing to spectral and/or morphological mismatches, the connection of these five GeV sources to the three LHAASO sources is not clear. This warrants deeper observations with HAWC and LHAASO, and a dedicated study of the modeling of the Galactic diffuse emission. Future CTAO observations with higher angular resolution are expected to deliver crucial information for the study of this region.

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.

Desk Editor's Note private letter to a colleague

This is a careful but incremental Fermi-LAT reanalysis that splits the region into five sources and flags unclear links to the LHAASO UHE objects, with the diffuse model as the main caveat the authors already note.

read the letter

This paper takes 17 years of Fermi-LAT data on the 52-55 degree strip and resolves the GeV emission into three point sources and two extended ones, improving on the 4FGL-DR4 description. One point source matches the known pulsar PSR J1932+1916 and another is offered as a new pulsar candidate; the extended sources get plausible leptonic or hybrid spectral interpretations tied to nearby PWNe and SNRs.

The work does the standard catalog-level analysis competently. Longer exposure lets them separate sources that were blended before, and the spectral comparisons to the LHAASO and other TeV data are laid out plainly. The conclusion that none of the five GeV sources match the three UHE sources cleanly follows from the reported positions and shapes.

The soft spot is the Galactic diffuse model. The region is crowded, the paper explicitly calls for a dedicated diffuse study, and the source count, locations, and extension measurements all rest on background subtraction that may leave residuals. That uncertainty directly affects how solid the mismatch arguments are.

This is for people who track specific Galactic plane patches and multiwavelength associations. A reader who needs the updated GeV breakdown for this longitude range will get a usable source list and spectra. It is honest about its limits and uses public data, so it deserves peer review; a referee can check the actual likelihood fits and model choices.

Referee Report

1 major / 1 minor

Summary. The paper analyzes >17 yr of Fermi-LAT data in the 52°<l<55° region containing three LHAASO UHE sources. It resolves the GeV emission into three point-like sources (J1925+1729P, J1930+1851P, J1932+1916P) and two extended sources (J1929+1732E, J1930+1826E), improving on 4FGL-DR4. J1932+1916P is identified as PSR J1932+1916; J1930+1851P coincides with PWN/SNR G54.1+0.3; spectral fits support leptonic/hybrid scenarios for the extended sources, but spectral/morphological mismatches preclude clear links to the three LHAASO sources. J1925+1729P is flagged as a possible new pulsar candidate. The work calls for deeper HAWC/LHAASO data, CTAO observations, and dedicated Galactic diffuse modeling.

Significance. If the source separation and spectral associations hold, the result supplies a detailed GeV view of a crowded field with multiple SNRs, PWNe and pulsars, offering concrete leptonic vs. hadronic interpretations and a candidate new gamma-ray pulsar. The explicit call for improved diffuse modeling is itself a useful contribution, as it quantifies a systematic limitation that affects many Galactic-plane studies.

major comments (1)

[Methods/Results] Methods and Results sections: The resolution into five distinct sources (three point-like, two extended) and the subsequent mismatch arguments with the LHAASO sources both rest on accurate subtraction of the Galactic diffuse emission. The manuscript itself states that “a dedicated study of the modeling of the Galactic diffuse emission” is warranted, indicating that residuals from the standard model may produce spurious detections or biased localizations/extensions in the 52°<l<55° region. This is load-bearing for the central claim of an improved description over 4FGL-DR4.

minor comments (1)

[Abstract] Abstract: The statement that the analysis “improves significantly on the description based on the 4FGL-DR4 catalog” is not accompanied by a quantitative metric (e.g., ΔTS or likelihood-ratio test) that would allow readers to judge the improvement.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thorough review and for identifying this key methodological consideration. We address the major comment below and will revise the manuscript accordingly to strengthen the presentation of systematic uncertainties.

read point-by-point responses

Referee: [Methods/Results] Methods and Results sections: The resolution into five distinct sources (three point-like, two extended) and the subsequent mismatch arguments with the LHAASO sources both rest on accurate subtraction of the Galactic diffuse emission. The manuscript itself states that “a dedicated study of the modeling of the Galactic diffuse emission” is warranted, indicating that residuals from the standard model may produce spurious detections or biased localizations/extensions in the 52°<l<55° region. This is load-bearing for the central claim of an improved description over 4FGL-DR4.

Authors: We agree that accurate modeling of the Galactic diffuse emission is essential for the source separation and for the subsequent arguments regarding mismatches with the LHAASO sources. The analysis employs the standard gll_iem_v07 model, and the manuscript already notes that a dedicated study of diffuse emission modeling in this region is warranted. This systematic uncertainty is indeed relevant to the robustness of the five-source description relative to 4FGL-DR4. In the revised manuscript we will expand the Methods section to provide additional detail on the diffuse model choice and any residual checks performed, and we will add a dedicated paragraph in the Results and Discussion sections that explicitly quantifies this limitation and qualifies the source detections, localizations, and mismatch conclusions accordingly. This will make the load-bearing nature of the diffuse subtraction transparent to readers while preserving the value of the current analysis as an improvement over the catalog under the standard model. revision: partial

Circularity Check

0 steps flagged

No circularity: data-driven Fermi-LAT source resolution

full rationale

The paper conducts a standard observational analysis of >17 yr Fermi-LAT data, performing likelihood fits to resolve five GeV sources (three pointlike, two extended) and comparing their positions/spectra to external catalogs (PSR J1932+1916, PWN/SNR G54.1+0.3, etc.). No equations, predictions, or uniqueness claims reduce by construction to fitted parameters or self-citations. Source associations use positional coincidence and spectral shape matching against independent data; the diffuse-model caveat is flagged as a limitation rather than a hidden assumption that forces the result. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms, or invented entities are stated. Standard gamma-ray astronomy assumptions (background subtraction, point-source vs extended morphology, leptonic/hadronic emission models) are implicit but not enumerated.

pith-pipeline@v0.9.1-grok · 6013 in / 1356 out tokens · 37821 ms · 2026-06-29T11:13:15.081054+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

3 extracted references · 1 canonical work pages

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work page doi:10.5281/zenodo.831784 2021

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work page doi:10.5281/zenodo.831784 2021