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

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.