Deep polarimetry study reveals double ring odd radio circle-like structures

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstraße 150, 44801 Bochum, Germany
² Ruhr Astroparticle and Plasma Physics Center (RAPP Center), 44780 Bochum, Germany
³ CSIRO Space and Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁴ SKA Observatory, SKA-Low Science Operations Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁵ Australia Telescope National Facility, CSIRO Space and Astronomy, PO Box 76 Epping, NSW 1710, Australia
⁶ Western Sydney University, Locked Bag 1797, Penrith South DC, NSW 2751, Australia
⁷ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, TAS 7001, Australia
¹⁰ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia

^⋆ Corresponding author: sam.taziaux@rub.de

Received: 24 June 2025
Accepted: 5 September 2025

Abstract

Context. New observations with the current generation of advanced radio interferometers, such as ASKAP and MeerKAT, have allowed observers to discover new classes of extended radio sources of unknown origins, including ‘odd radio circles’ (ORCs). These phenomena are detected exclusively in the radio continuum, with no clear counterparts of the structures at other wavelengths, making their physical nature and origin a subject of ongoing investigation.

Aims. To better understand these phenomena and their origin, we studied their radio continuum emission, spectral characteristics, and magnetic field properties.

Methods. This study presents a radio spectropolarimetry analysis of a newly discovered ORC (ORC J0356–4216) displaying a rare double-ring morphology. We used data from the MeerKAT L-band and from the ASKAP Evolutionary Map of the Universe (EMU) at 943 MHz.

Results. ORC J0356–4216 shows a symmetric double ring structure with a diameter of approximately 2′, corresponding to a physical size of 667.6 kpc, based on the redshift (0.494 ± 0.068) of its apparent host galaxy WISEA J035609.67–421603.5. The radio spectra of both rings are steep, with spectral indices of −1.18 ± 0.03 and −1.12 ± 0.05, and show no significant substructures. Equipartition magnetic field strengths with K₀ = 1 are estimated to be 1.82 μG and 1.65 μG for the respective rings. The degree of polarisation across the object ranges between 20−30%, further supporting a non-thermal origin.

Conclusions. The morphology and polarisation are broadly consistent with large-scale shocks driven by powerful starburst outflows. Nevertheless, the high degree of symmetry, coherent double-ring structure, and absence of internal substructures are features commonly associated with relic active galactic nucleus (AGN) lobes, making this scenario particularly compatible with the observed characteristics.