Gamma-Ray Emission from the Shell of Supernova Remnant W44 Revealed by the Fermi LAT

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Science  07 Jan 2010:
DOI: 10.1126/science.1182787


Recent observations of supernova remnants (SNRs) hint that they accelerate cosmic rays to energies close to ~1015 eV. However, the nature of the particles that produce the emission remains ambiguous. We report observations of SNR W44 with the Fermi Large Area Telescope at energies between 200 MeV and 300 GeV. The detection of a source with a morphology corresponding to the SNR shell implies that the emission is produced by particles accelerated there. The gamma-ray spectrum is well modeled with emission from protons and nuclei. Its steepening above ~GeV provides a probe with which to study how particle acceleration responds to environmental effects such as shock propagation in dense clouds, and how accelerated particles are released into interstellar space.