$\gamma$-Cygni supernova remnant in $\gamma$-rays: signatures of trapped and escaped Cosmic Rays

Abstract

We reanalyze 15 years of data recorded by the Fermi Large Area Telescope in a region around supernova remnant (SNR) $\gamma$-Cygni from 100 MeV to 1 TeV, and find that the spectra of two extended sources associated with the southeast radio SNR arc and the TeV VERITAS source can be described well by single power-laws with photon indices of $2.149\pm0.005$ and $2.01\pm0.06$, respectively. Combining with high resolution observations of surrounding gas, we model the emission in the hadronic scenario, where the $\gamma$-ray emission could be interpreted as escaped cosmic ray (CR) illuminating 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 MC mass between the escaped component and the trapped component in the two regions. We further analyze, in a potential pulsar halo region, the relationship between energy density $\varepsilon_{\rm{e}}$, spin-down power $\dot{E}$, and the $\gamma$-ray luminosity $L_{\gamma}$ of PSR J2021+4026. Our results indicate that the existence of a pulsar halo is unlikely. On the other hand, considering the uncertainty on the SNR distance, the derived energy density $\varepsilon_{\rm{e}}$ might be overestimated, thus the scenario of a SNR and a pulsar halo overlapping in the direction of the line of sight (LOS) cannot be ruled out.