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X-Ray Polarimetry Workshop SLAC, Stanford, California 9-11 February 2004 |
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Shell-type supernova-remnants as cosmic-ray sources: the X-ray/radio polarization connection
Stephen
Reynolds
North Carolina State University
steve_reynolds@ncsu.edu
(none)
Shell supernova remnants (SNRs) are generally accepted as the source of Galactic cosmic rays at least up to energies of order 3000 TeV, the "knee" in the cosmic-ray integrated spectrum. This conclusion is based on general arguments but has surprisingly weak direct observational support. In recent years, featureless X-ray spectra in a few SNRs have been plausibly interpreted as synchrotron emission, requiring the presence of electrons with energies of order 100 TeV, presumably accelerated in the remnant blast wave. However, this conclusion lacks direct confirmation. The detection of X-ray polarization from shell remnants would confirm a synchrotron origin. In addition, it could discern a nonthermal, synchrotron component in remnants whose spectra also show thermal line spectra. While radio polarization observations are straightforward, they can be rendered difficult to interpret by any of several related effects: Faraday rotation due to foreground plasma, depolarization due to internal Faraday rotation, and bandwidth depolarization due to differential Faraday rotation across an observing band. X-ray polarization observations are free of all Faraday effects and could characterize the properties of the narrower emission regions where the very highest-energy electrons should occur, giving crucial information on magnetic-field direction and degree of order in the acceleration regions. Comparison with radio polarimetry could then constrain the quantities that produce internal depolarization in the radio: the thermal electron density in the remnant and the line-of-sight magnetic-field component, otherwise inaccessible since it does not contribute to synchrotron emission. However, the poorer spatial resolution of foreseeable X-ray polarimeters can cause beam depolarization to a greater extent than in radio. I shall summarize the relative importance of these various effects, and show examples of particular theoretical models of nonthermal emission from SNRs to illustrate the potential of X-ray polarimetry for the study of remnants and particle acceleration.
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