Interacting Supernova Remnants: Cosmic Ray Accelerators revealed by Fermi
Abstract
Supernova remnants (SNRs) interacting with molecular clouds are potentially exciting systems in which to detect evidence of cosmic ray acceleration. The large reservoir of dense gas in the cloud acts as a target for particles accelerated by the supernova blastwave. Such systems should be prominent gamma-ray emission at GeV energies, allowing detailed studies of the nature of the underlying accelerated particle population, the environmental effects on particle acceleration, and the diffusion of the recently accelerated cosmic rays into the surrounding interstellar medium. Using the OH(1720 MHz) maser as an unambiguous tracer of shock interaction with dense gas, the first year of Fermi-LAT observations have been searched for counterparts to interacting SNRs at energies between 200 MeV and 300 GeV. Twelve sources are identified coincident with maser SNRs. Gamma-ray emission from these sources is well modeled by neutral pion decay produced after accelerated protons and nuclei collide with the dense ambient gas. Spectral steepening above 1 GeV is observed for several sources, giving evidence of environmental effects on the population of the highest energy cosmic rays. The total energy of accelerated particles is a estimated to be a few percent of the total supernova energy. This is more than an order of magnitude higher than the local cosmic ray density. The enhanced ionization rate from cosmic rays is sufficient to produce the non-equilibrium shock chemistry needed to explain the high columns OH in the post-shock gas that gives rise to OH(1720 MHz) masers. Extended OH enhancements throughout the post-shock gas suggest the enhanced cosmic rays permeate the shock interface with molecular clouds.
This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center.- Publication:
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AAS/High Energy Astrophysics Division #11
- Pub Date:
- November 2010
- Bibcode:
- 2010HEAD...11.2705H