Fermi large area telescope observations of the Cygnus loop supernova remnant

Abstract

We present an analysis of the gamma-ray measurements by the Large Area Telescope onboard the 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 0.2-100GeV. The gamma-ray spectrum shows a break in the range 2-3GeV. The gamma-ray luminosity is 1 × 1033ergs-1 between 1 and 100GeV, much lower than those of otherGeV-emitting SNRs. The morphology is best represented by a ring shape, with inner/outer radii 0°.7 ± 0°.1 and 1°.6 ± 0°.1. Given the association among X-ray rims, Hα 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. © 2011 The American Astronomical Society. All rights reserved.