Constraining leptonic and hadronic gamma-ray emission from HESS J1825-137 and its environment

Abstract

We present a broadband spectral analysis of the $\gamma$-ray emission from the pulsar wind nebula HESS~J1825$-$137, combining observations from Fermi Large Area Telescope (\textit{Fermi}-LAT), High Energy Stereoscopic System (H.E.S.S.), High-Altitude Water Cherenkov Observatory (HAWC), and Very Energetic Radiation Imaging Telescope Array System (VERITAS) across the $\sim 0.1$~GeV--$160$~TeV energy range. The spectral energy distribution is modelled under purely leptonic, purely hadronic, and lepto-hadronic scenarios using the \textsc{Naima} radiative modeling framework with Markov Chain Monte Carlo parameter estimation. Model comparison via the Bayesian Information Criterion reveals that the baseline GeV--TeV data favour a purely leptonic interpretation, while the inclusion of simulated Cherenkov Telescope Array Observatory (CTAO) observations or Large High Altitude Air Shower Observatory (LHAASO) ultra-high-energy (UHE; $E_{\gamma} \ge 100\,\mathrm{TeV}$) measurements shifts the preference toward models incorporating a hadronic component ($\Delta\mathrm{BIC} = -28.87$ and $-7.89$, respectively). The inferred electron energy budget for the baseline GeV--TeV dataset, $W_e = 4.25 \times 10^{48}$~erg, is consistent with previous estimates reported in the literature. The proton energy budget, $W_p \approx 2.5 \times 10^{48}$~erg, is energetically compatible with $pp$ interactions in the dense molecular environment adjacent to the nebula. These results demonstrate that precise spectral measurements above $\sim 10$~TeV, where Klein--Nishina suppression of inverse Compton emission creates a window for hadronic processes, are essential to establish the dominant emission mechanism in this source.