2025A&A...702A.219T

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.