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arXiv · 2610.41739

Gamma-Ray Emission from LS 5039: A 30-500 GeV Excess and Phase-Resolved Broadband Spectral Modeling

Xiaozhen Zhao·Jiandong Zhang·Pengfei Zhang
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Abstract

LS~5039 is a well-studied $γ$-ray binary exhibiting strong orbital modulation from GeV to TeV energies. We present a phase-resolved analysis of 17.3~yr of \textit{Fermi}-LAT observations and identify an additional $γ$-ray excess in the $\sim30$--$500$~GeV at a significance of $\sim7.9σ$. The phase-resolved spectra show pronounced orbital dependence above $\sim10$~GeV, with the inferior-conjunction more complex than the superior-conjunction. To investigate the broadband spectral energy distribution, we combine the \textit{Fermi}-LAT spectra with published H.E.S.S.\ and HAWC measurements and compare purely leptonic and lepto-hadronic models using the Bayesian information criterion. In both intervals, relativistic-Maxwellian and magnetospheric components dominate below $\sim10$~GeV, while shock-accelerated electrons account for much of the GeV--TeV continuum. The purely leptonic fits leave residuals at $30$--$500$~GeV and underpredict the HAWC measurements above $\sim100$~TeV. In the local one-zone intrabinary shock apex model, synchrotron cooling and Klein-Nishina suppression make the ultra-high-energy emission difficult to reproduce, although more complex leptonic scenarios remain possible. Adding a phenomenological proton component improves the broadband description, with stronger relative support during superior-conjunction than inferior-conjunction. However, the proton parameters remain weakly constrained and the fast-escape hadronic interpretation is energetically challenging, so a hadronic origin is not uniquely established. Future high-sensitivity, phase-resolved observations at multi-TeV and ultra-high energies will be required to test this interpretation.

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Xiaozhen Zhao, Jiandong Zhang, Pengfei Zhang. 2026-10-30. Gamma-Ray Emission from LS 5039: A 30-500 GeV Excess and Phase-Resolved Broadband Spectral Modeling. https://arxiv.org/abs/2610.41739

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