Investigating the Cosmic-ray Ionization Rate Near the Supernova Remnant IC 443 through H^(+)_(3) Observations

Creators: Indriolo, Nick; Blake, Geoffrey A.; Goto, Miwa; Usuda, Tomonori; Oka, Takeshi; Geballe, T. R.; Fields, Brian D.; McCall, Benjamin J.

Abstract

Observational and theoretical evidence suggests that high-energy Galactic cosmic rays are primarily accelerated by supernova remnants. If also true for low-energy cosmic rays, the ionization rate near a supernova remnant should be higher than in the general Galactic interstellar medium (ISM). We have searched for H^(+)_(3) absorption features in six sight lines which pass through molecular material near IC 443—a well-studied case of a supernova remnant interacting with its surrounding molecular material—for the purpose of inferring the cosmic-ray ionization rate in the region. In two of the sight lines (toward ALS 8828 and HD 254577) we find large H^(+)_(3) column densities, N(H^(+)_(3)) ≈ 3 × 10^(14) cm^(–2), and deduce ionization rates of ζ2 ≈ 2 × 10^(–15) s^(–1), about five times larger than inferred toward average diffuse molecular cloud sight lines. However, the 3σ upper limits found for the other four sight lines are consistent with typical Galactic values. This wide range of ionization rates is likely the result of particle acceleration and propagation effects, which predict that the cosmic-ray spectrum and thus ionization rate should vary in and around the remnant. While we cannot determine if the H^(+)_(3) absorption arises in post-shock (interior) or pre-shock (exterior) gas, the large inferred ionization rates suggest that IC 443 is in fact accelerating a large population of low-energy cosmic rays. Still, it is unclear whether this population can propagate far enough into the ISM to account for the ionization rate inferred in diffuse Galactic sight lines.

Additional Information

© 2010 American Astronomical Society. Received 2010 July 7; accepted 2010 September 24; published 2010 November 12. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors thank Steve Federman, Farhad Yusef-Zadeh, and the anonymous referee for helpful comments and suggestions. N.I. and B.J.M. are supported by NSF grant PHY 08-55633. G.A.B. is supported by NSF grant AST 07-08922. T.O. is supported by NSF grant AST 08-49577. T.R.G.'s research is supported by the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Australia, Brazil, Canada, Chile, the United Kingdom, and the United States of America. The work of B.D.F. was partially supported by the NASA Astrophysics Theory Program through award NNX10AC86G. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions. The Second Palomar Observatory Sky Survey (POSS-II) was made by the California Institute of Technology with funds from the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation.

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