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Lookup NU author(s): Dr Andrew Fletcher,
Professor Anvar Shukurov
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Narrow depolarized canals are common in maps of the polarized synchrotron emission of the Milky Way. Two physical effects that can produce these canals have been identified: the presence of Faraday rotation measure (RM) gradients in a foreground screen, and the cumulative cancellation of polarization known as differential Faraday rotation. We show that the behaviour of the Stokes parameters Q and U in the vicinity of a canal can be used to identify its origin. In the case of canals produced by a Faraday screen we demonstrate that, if the polarization angle changes by 90 degrees. across the canal, as is observed in all fields to-date, the gradients in RM must be discontinuous. Shocks are an obvious source of such discontinuities and we derive a relation of the expected mean separation of canals to the abundance and Mach number of supernova-driven shocks, and compare this with recent observations by Haverkorn, Katgert & de Bruyn. We also predict the existence of less common canals with polarization angle changes other than 90 degrees. Differential Faraday rotation can produce canals in a uniform magneto-ionic medium, but as the emitting layer becomes less uniform the canals will disappear. We show that for moderate differences in emissivity in a two-layer medium, of up to 1/2, and for Faraday depth fluctuations of standard deviation <= 1 rad, canals produced by differential rotation will still be visible.
Author(s): Fletcher A, Shukurov A
Publication type: Article
Publication status: Published
Journal: Monthly Notices of the Royal Astronomical Society
ISSN (print): 0035-8711
ISSN (electronic): 1365-2966
Publisher: Wiley-Blackwell Publishing Ltd.
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