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Cosmic-ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant

Nature Physics volume 2, pages 614619 (2006) | Download Citation

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Abstract

The acceleration of cosmic rays in our Galaxy by means of diffusive shock (Fermi) acceleration is believed to occur primarily in supernova remnants (SNRs). Despite considerable theoretical work, the precise details are still unknown, in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in—and affect the structure of—SNR shocks. However, for the past ten years, X-ray observatories such as ASCA (Advanced Satellite for Cosmology and Astrophysics) and, more recently, Chandra, XMM-Newton and Suzaku, have made it possible to image the keV-scale synchrotron emission produced by cosmic-ray electrons accelerated in SNR shocks. Here, we describe a spatially resolved spectroscopic analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the cutoff frequencies of electrons accelerated in the forward shock. We set upper limits on the diffusion coefficient and find locations where particles seem to be accelerated nearly as fast as theoretically possible (the Bohm limit).

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Acknowledgements

M.D.S. and G.E.A. thank T. Delaney and L. Rudnick for sharing unpublished radio spectral index data of Cas A to help us in determining a reasonable choice of Γ for the forward shock. This work was supported in part by NASA LTSA grant NAG5-9237 and the Five College Astronomy Department Fellowship program.

Author information

Affiliations

  1. Five College Astronomy Department, University of Massachusetts, 710 North Pleasant Street, Amherst, Massachusetts 01003-9305, USA

    • M. D. Stage
  2. MIT, Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA

    • G. E. Allen
    • , J. C. Houck
    •  & J. E. Davis

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Contributions

This research project was conceived and designed by all of the authors. The research and writing was primarily by M.D.S. and the scientific analysis and interpretation primarily by M.D.S. and G.E.A. Design of the spectral models was primarily by J.C.H. and G.E.A. Design and creation of the distributed spectral fitting and mapping software was primarily by J.C.H. and the new data reduction and processing tools were created primarily by J.E.D.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to M. D. Stage.

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https://doi.org/10.1038/nphys391

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