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The Global Magneto-Ionic Medium Survey (GMIMS): the brightest polarized region in the southern sky at 75 cm and its implications for Radio Loop II

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journal contribution
posted on 2023-05-21, 12:04 authored by Thomson, AJM, Landecker, TL, McClure-Griffiths, NM, John DickeyJohn Dickey, Campbell, JL, Carretti, E, Clark, SE, Federrath, C, Gaensler, BM, Han, JL, Haverkorn, M, Hill, AS, Mao, SA, Ordog, A, Pratley, L, Reich, W, Van Eck, CL, West, JL, Wolleben, M
Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS) southern sky polarization survey, covering 300-480 MHz at 81  arcmin resolution, we reveal the brightest region in the southern polarized sky at these frequencies. The region, G150-50, covers nearly 20 deg2⁠, near (l, b)~(150○, -50○). Using GMIMS-LBS and complementary data at higher frequencies (~0.6-30 GHz), we apply Faraday tomography and Stokes QU-fitting techniques. We find that the magnetic field associated with G150−50 is both coherent and primarily in the plane of the sky, and indicates that the region is associated with Radio Loop II. The Faraday depth spectra across G150-50 are broad and contain a large-scale spatial gradient. We model the magnetic field in the region as an expanding shell, and we can reproduce both the observed Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using QU fitting, we find that the Faraday spectra are produced by several Faraday dispersive sources along the line of sight. Alternatively, polarization horizon effects that we cannot model are adding complexity to the high-frequency polarized spectra. The magnetic field structure of Loop II dominates a large fraction of the sky, and studies of the large-scale polarized sky will need to account for this object. Studies of G150−50 with high angular resolution could mitigate polarization horizon effects, and clarify the nature of G150−50.


Publication title

Monthly Notices of the Royal Astronomical Society








School of Natural Sciences


Blackwell Publishing Ltd

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9600 Garsington Rd, Oxford, England, Oxon, Ox4 2Dg

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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2021 the author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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