Â鶹ÒùÔº

Astronomers from Ruhr University Bochum in Germany and elsewhere have conducted radio spectropolarimetric observations of a recently identified odd radio circle designated ORC J0356–4216. Results of the observational campaign, Sept. 5 on the arXiv pre-print server, shed more light on the nature of this object.

The so-called odd radio circles (ORCs) are mysterious gigantic rings of radio waves and their origin is still unexplained. They are highly circular and bright along the edges at radio wavelengths but they cannot be observed at visible, infrared or X-ray wavelengths. To date, only a few objects of this type have been identified, hence very little is known about their nature.

ORC J0356–4216 was identified in October 2023 with the MeerKAT radio telescope and shortly after its discovery, a group of astronomers led by Ruhr University Bochum's Sam Taziaux, performed radio spectropolarimetry of this source using the Australian SKA Pathfinder (ASKAP) and MeerKAT to investigate its properties and nature.

"We present the first radio continuum polarimetry study of a new ORC-like system, which we dub ORC J0356–4216," the researchers write in the paper.

The study found that ORC J0356–4216 exhibits a symmetric double ring structure with a diameter of approximately two arcminutes, which corresponds to a physical size of 2.18 million light years. The ORC is located at a redshift of 0.494, in the galaxy WISEA J035609.67–421603.5, and has the shape of a shell or a shock front.

By analyzing the collected data, the astronomers found that the rings of ORC J0356–4216 have integrated flux density at 943 MHz of 4.07 mJy and 2.82 mJy, while at 1.28 GHz, these values are 2.77 mJy and 1.98 mJy. The radio spectra of both rings turned out to be steep, with spectral indices of -1.18 and -1.12, and show no significant substructures.

Moreover, the total equipartition magnetic field of the rings reached values of 1.82 and 1.65 µG. The polarized intensity of the rings was measured to be 0.47 mJy, which corresponds to a degree of polarization of 20–30% through the rings. It was noted that the magnetic field orientations follow the rings pointing tangential to the rings.

Trying to explain the origin of ORC J0356–4216, the authors of the paper see two hypotheses as the most plausible. They assume that it may be a remnant from a past active galactic nucleus (AGN) phase or the result of a large-scale shock wave, such as those generated by galaxy group interactions or mergers.

"In the case of ORC J0356–4216, the observed double-lobed morphology and polarization characteristics are more readily explained by relic emission from previous AGN activity or jet-driven outflows. The WISE colors are consistent with the host being an AGN in an elliptical galaxy. Given the available data, an AGN-related origin appears to be the more consistent interpretation," the researchers conclude.