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

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 $η_p \sim 0.14$ 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 $α= 1.05$, within an ambient of density, $1~\mathrm{cm}^{-3}$.