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

arXiv:2601.00692 · astro-ph.HE · Submitted 2026-01-02 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+3755".

Jocelyn: Recent observations of sub-PeV gamma-rays from the halo of pulsar PSR J0622+3749 provide an opportunity to investigate alternative emission mechanisms beyond leptonic models.

Vera: First, who's behind it and why it matters.

Title and authors: Vera: So, we're starting by looking at the title of "Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+three thousand seven hundred fifty-five" and it immediately tells us what the core investigation is about <ref:2601.00692#pg0,Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+3755>. Jocelyn It really focuses on exploring whether the gamma-ray signals we're seeing from this specific source, LHAASO J0621+three thousand seven hundred fifty-five could be explained by a hadronic process instead of just the standard leptonic ones <ref:2601.00692#pg0>.

Subrahmanyan: From a theoretical standpoint, exploring alternative emission mechanisms is crucial because it helps us constrain the physics occurring in these extreme environments around pulsars. Vera Exactly, and this paper proposes using the proton-proton interaction channel to explain that spectrum we observe, which is quite different from what we usually expect from electrons scattering photons.

Jocelyn: I’m curious about what that means for us observing the sky; does it suggest a fundamentally different physical process is at play in these pulsar halos? Subrahmanyan It suggests that cosmic ray protons interacting with ambient matter or photons are creating those high-energy gamma-rays through neutral pion decay, which is a distinct pathway from inverse Compton scattering.

Vera: That's the main point of the paper; they’re looking at how we can explain the observed spectrum using protons rather than just leptons. Jocelyn It seems like they are trying to bridge a gap between what we see in gamma-rays and what we know about cosmic ray acceleration near pulsars.

The paper's summary: Vera: Moving on, let’s talk about the main findings summarized in "Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+three thousand seven hundred fifty-five <ref:2601.00692#pg0,Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+3755>." Essentially, they found that a hadronic model fits the observed gamma-ray spectrum well, spanning from TeV up to two hundred TeV <ref:2601.00692#pg1>. Jocelyn So, if I understand correctly, the authors concluded that this proton-proton interaction scenario provides a plausible explanation for the sub-PeV gamma-rays we are measuring in these halos.

Subrahmanyan: The summary points out that they require a specific proton luminosity to match those observations, finding that eta p about zero point one four of the spin-down luminosity of PSR J0622+three thousand seven hundred forty-nine is needed, assuming the protons diffuse in a one-zone superdiffusive environment with an index of alpha = one point zero five and an ambient density of one cm-three. Vera That requirement for the proton luminosity is a big piece of information because it links the gamma-ray observation directly to the pulsar's rotational energy.

Jocelyn: That makes sense; so they aren't just guessing where the emission comes from, but they are quantifying exactly how much accelerated proton flux is needed to generate that specific spectrum. Vera And they’ve done some detailed modeling, including solving the GAMERA framework for particle transport and fitting parameters like alpha p and N zero using data from HAWC and LHAASO, while also keeping an eye on Fermi-LAT upper limits <ref:2601.00692#pg1>.

Subrahmanyan: The modeling involves a complex time-dependent proton transport equation, where the injection spectrum is defined by Q p(N p, gamma p, t) = N zero gamma-alpha p p (-gamma p/gamma cut), which shows how protons enter and evolve in the environment <ref:2601.00692#pg1>. Vera It’s quite detailed; they’re not just throwing numbers at the problem but building a full physical model of how these particles move and interact over time.

Jocelyn: And then there's the connection to neutrinos, which is an important addition to this hadronic picture because it suggests secondary particle production from those interactions. Vera Right, they calculated a neutrino differential flux using Equation seven based on the pi zero decay channel, and that prediction is quite interesting for future detectors <ref:2601.00692#pg1>.

The paper's improvements: Vera: Now we’re looking at how this work advances the field by discussing its improvements, and it seems the authors highlight several things they did to make their model more robust. Jocelyn I see they specifically addressed potential issues with previous studies, like accounting for a source that wasn't fully modeled before, which is a good step forward.

Subrahmanyan: They also included analysis of different observational constraints, incorporating both upper limits from Fermi-LAT and point source flux measurements from instruments like VERITAS and HAWC to optimize the spectral parameters. Vera That multi-instrument approach really strengthens the model because it doesn't rely on just one piece of data, which is something I always look for in astrophysical research.

Jocelyn: They also used a specific diffusion coefficient, D alpha(E p) = D zero E p-one/three where D zero is related to the diffusion parameter, which ties the transport model more directly into the physical environment of the halo <ref:2601.00692#pg1>. Vera That inclusion of a specific diffusion physics scenario helps ground their abstract particle transport equation in real astrophysical conditions, showing how protons move through that ambient medium.

Subrahmanyan: Furthermore, they calculated long cooling timescales for these protons, finding that t pp = one/(n H sigma ppc) is around twenty-two thousand kyr at an energy of E p = one hundred TeV, which gives us a good idea of how long these accelerated protons can remain energetic in the halo. Vera That timescale information is critical because it tells us whether the emission is steady or transient, and this calculation shows they are relatively long for this system.

Conclusion: Jocelyn: So, wrapping up the discussion on "Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+three thousand seven hundred fifty-five" the authors present a self-consistent picture where the hadronic proton interaction channel successfully explains the observed gamma-ray spectrum across that wide energy range <ref:2601.00692#pg0,Hadronic Origin of Sub-PeV Gamma-Ray Emission from LHAASO J0621+3755>. Vera It really seems like they’ve managed to create a framework that connects particle acceleration, transport physics, and observational data in one coherent way for this source.

Subrahmanyan: The main implication is that this scenario provides a viable alternative explanation to the standard leptonic models for these sub-PeV gamma-rays, especially when considering the constraints from lower energy observations. Jocelyn And they also provided some concrete predictions, like the neutrino flux calculation using Equation seven which gives us a clear target for next-generation neutrino observatories <ref:2601.00692#pg1>.

Vera: It’s exciting because it moves us toward testing this idea with multi-messenger astronomy; if we can detect those predicted neutrinos, it would confirm this hadronic interpretation. Jocelyn I agree; linking gamma-ray observations directly to neutrino fluxes is a powerful way to validate these complex models.

Subrahmanyan: This work opens up avenues for future modeling, allowing us to systematically test how different environmental densities and magnetic fields might alter the resulting gamma-ray spectrum and the predicted neutrino flux. Vera So, in summary, this paper gives us a detailed roadmap for using hadronic physics to interpret VHE emission from pulsar halos. Jocelyn We're really looking forward to seeing how these results shape our understanding of high-energy astrophysics moving forward.

Department of Physics, Indian Institute of Technology Jodhpur

astro-ph.HE

Submitted: 2026-01-02

Updated: 2026-05-15

Comments: 8 pages, 5 figures, 1 table

Journal ref: Phys. Rev. D 113, 123016 (2026)

DOI: 10.1103/q1m6-95c3

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 79/100

The gist: Recent observations of sub-PeV gamma-rays from the halo of pulsar PSR J0622+3749 provide an opportunity to investigate alternative emission mechanisms beyond leptonic models.

Key concepts

Leptonic vs. Hadronic Scenarios: Leptonic models suggest gamma-rays come from electrons upscattering ambient light (inverse Compton scattering). The hadronic model proposes that high-energy gamma-rays result from neutral pion decay, which occurs when accelerated protons collide with ambient photons or matter.
Proton Luminosity ($\eta_p$): This is the fraction of the pulsar's spin-down energy carried by cosmic ray protons. The study found that $\eta_p \sim 0.14$, meaning about 14% of the pulsar's rotational energy goes into accelerating these protons to explain the observed gamma-ray flux.
Neutral Pion Decay ($\pi^0$ decay): When accelerated protons interact with ambient photons or matter, they create neutral pions ($\pi^0$). These pions are unstable and quickly decay into high-energy gamma-rays. This process is a key mechanism proposed in the hadronic scenario for generating TeV emission.
Proton Transport Equation: This mathematical equation describes how the distribution of accelerated protons changes over time as they diffuse through the environment. It accounts for particle injection, energy losses, and escape from the pulsar wind nebula.

Terminology

Power Law — 4× in this paper
alpha — 4× in this paper · explained in 1 episode
Inverse Compton (IC) scattering — 3× in this paper · explained in 2 episodes
Very High Energy — 3× in this paper
Diffusion Model — 2× in this paper · explained in 8 episodes
Abundance — 1× in this paper · explained in 1 episode
Adam — 1× in this paper · explained in 2 episodes
Core — 1× in this paper · explained in 2 episodes
Fermi-Large Area Telescope — 1× in this paper
Multi-messenger Astronomy — 1× in this paper · explained in 3 episodes

Summary

Recent observations of sub-PeV gamma-rays from the halo of pulsar PSR J0622+3749 provide an opportunity to investigate alternative emission mechanisms beyond leptonic models. This work explores a hadronic scenario through the proton-proton (pp) interaction channel to explain the observed gamma-ray spectrum, ranging from TeV up to 200 TeV.

The Gist

The required CR proton luminosity is found to be ηp ∼ 0.14 of the spin-down luminosity of PSR J0622+3749, assuming protons propagate in a one-zone superdiffusive environment characterized by a diffusion index α = 1.05 within an ambient density of 1 cm−3.

Leptonic vs. Hadronic Scenarios

The standard interpretation for GeV-TeV gamma-rays from pulsar wind nebulae (PWNe) involves the leptonic channel, where ultrarelativistic electrons upscatter ambient photons via inverse Compton (IC) scattering. This process accounts for synchrotron radiation at lower wavelengths and GeV-TeV emission through IC scattering of ambient photon fields by lepton pairs. However, the absence of X-ray emission constrains the magnetic field in the halo region to ≲ 1 µG when interpreted within a leptonic synchrotron scenario.

Hadronic Mechanism and Proton Dynamics

The paper proposes that sub-PeV gamma-rays originate from neutral pion decay resulting from interactions of accelerated protons with ambient photons (pγ) or matter (pp). The key elements of this hadronic model include:

Protons accumulate in the PWN due to slow diffusion and no appreciable energy losses, unlike leptons.
High-energy gamma-rays originate from the decay of neutral pions produced in interactions of accelerated protons with ambient photons (pγ) or matter (pp).
For old or middle-aged PWNe like PSR J0622+3749 (estimated age ≈ 207.8 kyr), the ambient photon fields are typically characterized by temperatures below 5000◦ K, which suppresses the efficiency of the pγ channel in producing TeV gamma-rays in their halos.
The proton cooling time scale is long: tpp = 1/(nHσppc) results in ∼ 22000 kyr at Ep = 100 TeV, and tp,sync = 1.5 × 10 15 kyr B−2 µGEp,100TeV [38].

Modeling the Proton Transport

The particle transport equation for protons is solved using the GAMERA framework [26], incorporating a time-dependent one-zone Kolmogorov diffusion scenario. Key aspects of this modeling include:

(1) Particle Injection Spectrum:

Qp(Np, γp, t) = Np(γp) = N0γ−αp p exp − γp / γcut, where N0 is the normalisation factor and αp is the energy spectral index.

(3) Proton Transport Equation:

∂Np(γp)/∂t = Qp(Np, γp, t) − ∂/∂γp(bNp(γp)) − Np(γp)/tesc(Ep, α).

(4) Escape Time Scale:

tesc(Ep, α) = R squared / (2Dα(Ep)) 1/α. The diffusion coefficient is considered Dα(Ep) = D0 Ep-1/3 where D0 is related to the diffusion parameter.

Parameter Fitting and Results

The parameters αp and N0 are optimized by modeling the observed VHE gamma-rays from HAWC and LHAASO, while considering upper limits from Fermi-LAT and VERITAS. The model fitting requires that ηp = 0.14, which is the fraction of the spin-down luminosity carried by protons to explain the observations. The resulting proton luminosity is calculated as Lp = Z γp,max γp,min dγp/dγpmpc squared Qp(Np, γp, t).

Neutrino Predictions

The pp interaction channel leads to the creation of secondary neutral pions (π0) and charged pions (π±), which subsequently decay into gamma-rays and neutrinos. The neutrino differential flux is calculated using:

E2ν dNν/dEν = 2/(3E2γ) dNγ/dEγ, where Eγ = 2Eν. For LHAASO J0621+3755, the corresponding neutrino differential flux without accounting for parameter errors can be calculated using:

dNν/dEν ∼ 7.5 × 10−18 Eν-2.92 TeV−1 cm−2 s−1 (7).

Improvements for AI systems

Here are the specific improvements that could be made to AI systems based on the insights from this scientific paper, along with what those improved AI systems could achieve:

The paper proposes a comprehensive, multi-messenger (gamma-ray + neutrino) hadronic model incorporating particle transport equations (GAMERA framework) and diffusion physics.
The proposed model requires fitting parameters like proton luminosity fraction to spin-down luminosity, diffusion indices, and environmental densities to match observations across different energy bands (TeV to sub-PeV).

AI System Improvement: Development of a Hadronic Astrophysical Simulator (HAS) Agent.

Specifically, the AI system can be improved in the following ways:

The system should be trained on the physics described by Equation 3 (the time-dependent proton transport equation) and Equation 5 (the analytical form for neutrino flux from neutral pion decay).
The system must incorporate a parameter fitting routine that optimizes the model against multi-wavelength data constraints (LHAASO, HAWC, Fermi-LAT) simultaneously, accounting for both leptonic and hadronic channels.
The system should be capable of simulating the time evolution of cosmic ray/hadronic populations within a specific astrophysical environment (like a pulsar wind nebula halo).

Improved AI System Capabilities:

The HAS Agent can perform Hypothesis Testing by taking observational data (e.g., gamma-ray flux from LHAASO) and predicting the necessary proton luminosity fraction required to explain it, allowing researchers to determine if a hadronic scenario is physically plausible versus a leptonic one.
It can simulate the expected neutrino flux at Earth (using Equation 7) based on a given proton model, providing a testable prediction for detectors like IceCube, thus linking sub-PeV gamma-ray observations directly to high-energy neutrino astronomy.
The system can rapidly assess the impact of changing environmental parameters (e.g., ambient density, magnetic field strength) on the resulting TeV gamma-ray spectrum and the calculated neutrino flux, enabling what-if scenario analysis for different astrophysical environments (e.g., comparing a high-density PWN vs. a low-density halo).
It can handle complex spectral modeling by integrating constraints from different observational instruments (Fermi-LAT upper limits, VERITAS/HAWC flux points) into a single, self-consistent physical model, moving beyond simple single-channel interpretations.