Fermi Large Area Telescope Observations of the Cygnus Loop Supernova Remnant

TL;DR

This paper analyzes Fermi LAT gamma-ray data of the Cygnus Loop supernova remnant, revealing a ring-shaped morphology, a spectral break at 2-3 GeV, and suggesting gamma rays originate from hadronic interactions near the shock regions.

Contribution

First detailed gamma-ray analysis of the Cygnus Loop SNR with spectral and morphological characterization, supporting hadronic origin of gamma rays.

Findings

Gamma-ray emission detected in 0.2-100 GeV range.

Spectral break observed at 2-3 GeV.

Gamma-ray morphology is ring-shaped with specific radii.

Abstract

We present an analysis of the gamma-ray measurements by the Large Area Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the region of the supernova remnant(SNR) Cygnus Loop(G74.0$-$8.5). We detect significant gamma-ray emission associated with the SNR in the energy band 0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The gamma-ray luminosity is $\sim$ $1 \times 10^{33}$erg s$^{-1}$ between 1--100 GeV, much lower than those of other GeV-emitting SNRs. The morphology is best represented by a ring shape, with inner/outer radii 0$^\circ$.7 $\pm$ 0$^\circ$.1 and 1$^\circ$.6 $\pm$ 0$^\circ$.1. Given the association among X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays originate in interactions between particles accelerated in the SNR and interstellar gas or radiation fields adjacent to the shock regions. The decay of neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar gas provides a reasonable explanation for the gamma-ray spectrum.