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arxiv: 2601.00692 · v2 · pith:3AMLYZHJnew · submitted 2026-01-02 · 🌌 astro-ph.HE

Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+3755

Sonali Sahoo , Ankan Roy , Kritika Yadav , Reetanjali Moharana This is my paper

Pith reviewed 2026-05-21 15:53 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords hadronic gamma rayspulsar halossub-PeV emissioncosmic ray protonsLHAASOproton-proton interactionssuperdiffusionPSR J0622+3749
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The pith

Sub-PeV gamma rays from the pulsar halo LHAASO J0621+3755 arise from cosmic ray protons colliding with ambient gas, using 14 percent of the pulsar's spin-down power.

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

The paper tests whether the gamma-ray spectrum observed from roughly 7 TeV to 200 TeV around PSR J0622+3749 can be produced through hadronic processes. It calculates the proton luminosity needed to match the data via proton-proton collisions that create neutral pions decaying into gamma rays. The model incorporates superdiffusive proton transport in a uniform low-density medium. A reader would care because this offers a concrete alternative to the usual leptonic explanation and shows that a modest share of the pulsar's energy budget is enough to sustain the emission.

Core claim

The observed gamma-ray spectrum from LHAASO J0621+3755 between approximately 7 TeV and 200 TeV is reproduced by neutral pion decay following cosmic ray proton interactions with ambient gas. The required proton luminosity equals 0.14 times the spin-down luminosity of PSR J0622+3749 when the protons propagate in a one-zone superdiffusive environment with diffusion index 1.05 inside gas of density 1 cm^{-3}.

What carries the argument

Proton-proton interaction channel producing neutral pions that decay into gamma rays, under one-zone superdiffusive proton propagation with diffusion index α = 1.05.

Load-bearing premise

The protons travel through a uniform gas density of exactly one particle per cubic centimeter while diffusing with an index of precisely 1.05 in a single zone.

What would settle it

A direct measurement of the gas density in the emission region that differs substantially from 1 cm^{-3}, or evidence that the energy dependence of diffusion deviates from index 1.05, would force a different proton luminosity and spoil the spectral match.

Figures

Figures reproduced from arXiv: 2601.00692 by Ankan Roy, Kritika Yadav, Reetanjali Moharana, Sonali Sahoo.

Figure 1
Figure 1. Figure 1: FIG. 1: SED of pulsar PSR J0622+3749/4FGL [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Sky map for the region around RA = 95 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Time evolution of the proton energy [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: The Fermi-LAT upper limits for the PL and [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
read the original abstract

Very High Energy (VHE) gamma-rays in pulsars, and their surrounding halos, are interpreted to originate from the leptonic channel, electromagnetic interactions through electron inverse Compton (IC) scattering. In the hadronic scenario, TeV-PeV gamma-rays are generated from the decay of neutral pions, which are produced from cosmic rays(CR) protons interacting with the ambient medium. Recent observations of sub-PeV gamma-rays from the halo of the pulsar PSR J0622+3749 by the Large High Altitude Air Shower Observatory Kilometer-Square Array (LHAASO-KM2A) provide an opportunity to investigate the underlying emission mechanisms. Previous studies have shown that the observed emission can be consistently explained within a leptonic framework by the slow diffusion of electrons. In this work, we explore an alternative explanation based on the hadronic scenario through the proton-proton ($pp$) interaction channel, incorporating the observation of VHE gamma-rays at 7 TeV by the High-Altitude Water Cherenkov detector (HAWC). To model the observed gamma-ray spectrum, ranging from $\sim 7~\mathrm{TeV}$ up to $200~\mathrm{TeV}$, the required CR proton luminosity is found to be $\eta_p \sim 0.14$ of the spin-down luminosity of PSR J0622+3749. This scenario assumes that protons propagate in a one-zone superdiffusive environment, characterized by a diffusion index $\alpha = 1.05$, within an ambient of density, $1~\mathrm{cm}^{-3}$.

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

2 major / 2 minor

Summary. The paper claims that the sub-PeV gamma-ray emission from LHAASO J0621+3755, associated with PSR J0622+3749, can be explained by a hadronic pp-interaction scenario. In a one-zone superdiffusive model with diffusion index α = 1.05 and ambient density 1 cm^{-3}, a cosmic-ray proton luminosity fraction η_p ∼ 0.14 of the pulsar's spin-down power reproduces the observed spectrum from ∼7 TeV to 200 TeV, providing an alternative to the leptonic interpretation.

Significance. If the modeling holds, the result demonstrates that hadronic channels can account for VHE emission in pulsar halos with a plausible fraction of the available power, broadening interpretations of LHAASO sources beyond purely leptonic models and highlighting the role of uncertain transport physics in these environments.

major comments (2)
  1. [Abstract / Modeling] Abstract and modeling section: The central result η_p ∼ 0.14 is obtained only after fixing α = 1.05 and n_H = 1 cm^{-3}; no derivation of these values from independent constraints (e.g., radio or X-ray data on the halo) or sensitivity tests under modest variations (α = 0.8–1.2 or n_H = 0.1–10 cm^{-3}) are shown, so the required luminosity fraction is not demonstrated to be robust.
  2. [Modeling] Modeling section: No goodness-of-fit metrics (χ², p-value), parameter uncertainties, or direct comparison of the predicted spectrum to the LHAASO-KM2A and HAWC data points are reported, making it impossible to assess whether the hadronic model actually reproduces the spectrum better than or comparably to the leptonic alternative mentioned in the introduction.
minor comments (2)
  1. [Abstract] The abstract states the gamma-ray range as ∼7 TeV up to 200 TeV; clarify whether the upper end is an observed detection or an extrapolation of the model.
  2. [Introduction] Notation: Define η_p explicitly on first use and specify whether it is the time-integrated or instantaneous luminosity fraction.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which help clarify the presentation of our hadronic modeling for the sub-PeV emission associated with PSR J0622+3749. We respond point by point to the major comments below, agreeing where revisions are warranted to improve robustness and quantitative assessment.

read point-by-point responses

  1. Referee: [Abstract / Modeling] Abstract and modeling section: The central result η_p ∼ 0.14 is obtained only after fixing α = 1.05 and n_H = 1 cm^{-3}; no derivation of these values from independent constraints (e.g., radio or X-ray data on the halo) or sensitivity tests under modest variations (α = 0.8–1.2 or n_H = 0.1–10 cm^{-3}) are shown, so the required luminosity fraction is not demonstrated to be robust.

    Authors: The value α = 1.05 was adopted to represent mildly superdiffusive transport consistent with theoretical expectations for cosmic-ray propagation in magnetized turbulence around pulsar halos, while n_H = 1 cm^{-3} is a standard average density for the Galactic interstellar medium at the source's location. We acknowledge that the original manuscript does not include explicit sensitivity tests or derivations from multi-wavelength constraints. In the revised manuscript we will add a dedicated subsection presenting results for α in the range 0.8–1.2 and n_H in the range 0.1–10 cm^{-3}, showing that η_p remains below ∼0.5 for plausible variations. Radio and X-ray data capable of independently constraining these parameters for this specific halo are not available in the current literature; we will note this limitation explicitly and discuss how future observations could provide such constraints. revision: partial

  2. Referee: [Modeling] Modeling section: No goodness-of-fit metrics (χ², p-value), parameter uncertainties, or direct comparison of the predicted spectrum to the LHAASO-KM2A and HAWC data points are reported, making it impossible to assess whether the hadronic model actually reproduces the spectrum better than or comparably to the leptonic alternative mentioned in the introduction.

    Authors: We agree that quantitative goodness-of-fit measures and direct data comparison are necessary for a rigorous evaluation. In the revised modeling section we will report the χ² per degree of freedom and associated p-value for the fit to the combined LHAASO-KM2A and HAWC spectral points, together with the uncertainty on the best-fit η_p obtained from the minimization. We will also include a figure overlaying the model spectrum on the observed flux points with error bars. For the leptonic comparison, we will expand the discussion to provide a side-by-side assessment of how both scenarios reproduce the spectrum, emphasizing the distinct transport physics assumptions while noting that the hadronic channel offers a viable alternative with η_p ∼ 0.14. revision: yes

Circularity Check

0 steps flagged

No significant circularity: standard hadronic modeling fit under explicit assumptions

full rationale

The paper models the observed 7–200 TeV gamma-ray spectrum using a hadronic pp-interaction scenario in a one-zone superdiffusive environment. It explicitly assumes a diffusion index α = 1.05 and ambient density n_H = 1 cm^{-3}, then determines the proton luminosity fraction η_p ∼ 0.14 needed to reproduce the data. This constitutes a conventional parameter fit to match observations rather than any first-principles derivation or prediction that reduces to the inputs by construction. No self-definitional steps, fitted inputs relabeled as predictions, load-bearing self-citations, uniqueness theorems, or ansatz smuggling appear in the derivation chain. The result is the direct output of the chosen transport and interaction parameters applied to the spectrum; the model remains falsifiable by varying α or n_H and checking fit quality against external benchmarks.

Axiom & Free-Parameter Ledger

3 free parameters · 1 axioms · 0 invented entities

The central claim rests on three fitted parameters (proton luminosity fraction, diffusion index, ambient density) and the standard assumption that pulsar-accelerated protons produce neutral pions via pp collisions whose decay yields the observed gamma rays. No new particles or forces are introduced.

free parameters (3)
  • η_p = 0.14
    Fraction of pulsar spin-down luminosity converted to cosmic-ray protons; value 0.14 chosen to match the observed gamma-ray flux.
  • α = 1.05
    Diffusion index for superdiffusive proton propagation; value 1.05 selected to reproduce the spectral shape.
  • ambient density = 1 cm^{-3}
    Gas density in the emission region; fixed at 1 cm^{-3} to enable the required interaction rate.
axioms (1)
  • domain assumption Protons accelerated by the pulsar interact with ambient gas via pp collisions to produce neutral pions that decay into gamma rays.
    Invoked in the abstract when stating the hadronic scenario; this is a standard assumption in high-energy astrophysics but is not derived within the paper.

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