γ-Cygni supernova remnant in γ-rays: Signatures of trapped and escaped cosmic rays

¹ Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 201210, PR China
² School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, PR China
³ School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, PR China
⁴ Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China

^★ Corresponding author: gwenael.giacinti@sjtu.edu.cn

Received: 7 December 2024
Accepted: 4 July 2025

Abstract

We reanalyzed 15 years of data recorded by the Fermi Large Area Telescope in a region around supernova remnant (SNR) γ-Cygni from 100 MeV to 1 TeV. We found that the spectra of two extended sources associated with the southeast radio SNR arc and the TeV VERITAS source are well described by single power laws with photon indices of 2.149 ± 0.005 and 2.01 ± 0.06, respectively. Combining this with high-resolution observations of the surrounding gas, we modeled the emission in the hadronic scenario, in which the γ-ray emission is interpreted as escaped cosmic rays (CRs) that illuminate a nearby molecular cloud (MC) plus an ongoing shock-cloud interaction component. In this scenario, the difference between the two GeV spectral indices is due to the different ratios of the MC mass between the escaped component and the trapped component in the two regions. We further analyzed in a potential pulsar halo region the relation between energy density ε_e, the spin-down power Ė, and the γ-ray luminosity L_γ of PSR J2021+4026. Our results indicate that a pulsar halo is unlikely. On the other hand, considering the uncertainty on the SNR distance, the derived energy density ε_e might be overestimated, and the scenario of an SNR and a pulsar halo that overlap in the direction of the line of sight therefore cannot be ruled out.