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

TL;DR

This work investigates the hadronic origin of sub-PeV gamma rays from the PSR J0622+3749 halo by evaluating proton-proton interactions in a diffusion-dominated environment. Using time-dependent proton injection tied to the pulsar’s spin-down luminosity and a one-zone superdiffusion framework implemented in GAMERA, the authors show that an injected proton fraction of $\eta_p=0.1$ can reproduce the LHAASO–HAWC TeV–sub-PeV observations. They predict associated neutrino fluxes and IceCube event rates, finding them small but potentially detectable with next-generation detectors, thereby providing a falsifiable test of the hadronic scenario. The analysis also clarifies that Fermi-LAT GeV data do not support a leptonic origin for the TeV halo and that GeV emission from the halo is not required, strengthening the case for hadronic processes in this system.

Abstract

Very High Energy (VHE) gamma rays are primarily estimated to arise from high-energy electromagnetic interactions in pulsars and their halo through electron inverse Compton (IC) scattering. Hadronic channels like neutral pion decay have also been proposed to produce TeV-PeV gamma rays from the Pulsar halo. The neutral pions are expected to be generated from cosmic ray (CR) protons interacting with the ambient/cloud. The recent observations of sub-PeV gamma rays from the halo of pulsar PSR J0622+3749 by the Large High Altitude Air Shower Observatory Kilometre-Square Array (LHAASO-KM2A) detector provide a platform to explore different channels of their production. Previous studies support consistency with the leptonic modeling of the halo, which attributes its origin to slow diffusion in the interstellar medium. In this work, we investigated the possibility of proton-proton channel as the origin of these photons. To explain the observed gamma rays with energy $\sim 4$ TeV by the High-Altitude Water Cherenkov (HAWC) telescope till 200 TeV by the LHAASO observatory, one requires the CR proton luminosity to be $η_p\sim 0.1$ of the pulsar PSR J0622+3749 spin-down luminosity. In this case, we have considered the protons propagating in a one-zone superdiffusion environment, specifically $α= 1$ in a cloud of gas density 1 per cm$^{3}$.

Figures (5)

• Figure 1: SED of pulsar PSR J0622+3749/4FGL J0622.2+3749 with Fermi-LAT.
• Figure 2: SED of bcu 4FGL J0620.3+3804 with Fermi-LAT; the uncertainty is represented by a butterfly-shaped shaded region.
• Figure 3: Sky map for the region around RA = $95.47^\circ$ Dec = $37.92^\circ$ for two logarithmically equi-spaced energy bins between 300 MeV to 300 GeV (300 MeV-9.487 GeV and 9.487 GeV-300 GeV, respectively, left and right). The magenta diamond marks the location of the 3HWC J0621+382. The green square and circle denote the best fit and 1 $\sigma$ range of the location of the LHAASO J0621+3755 source. The cyan circle shows the locations of two 4FGL sources.
• Figure 4: Time evolution of the proton energy spectrum in the PWN J0622+3749. The dotted, dashed, and solid curves correspond to $t = 1\,\mathrm{s}$, $10\,\mathrm{kyr}$, and $207.8\,\mathrm{kyr}$, respectively.
• Figure 5: The Fermi-LAT upper limits for the PL and SupExp models, LHAASO J0621+3755, are presented within the energy range of $15\,\mathrm{GeV}$ to $500\,\mathrm{GeV}$. The X-ray (XMM-Newton) and gamma-ray (VERITAS) observations are taken from VERITAS:2025xjd. A solid line indicates the modeling for the proton-proton (pp), and the $e^{\pm}$ diffusion model is shown with a dash-dotted line, as taken from PhysRevLett.126.241103.