A Compact Radio Ring with a Diffuse Envelope in LOFAR: Odd Radio Circle or Distinct Phenomenon?

TL;DR

This paper reports the discovery of a compact ORC-like radio source in LOFAR data, analyzing its properties and environment, and discusses its possible origins and implications for the understanding of diffuse radio sources.

Contribution

The study presents the most compact ORC candidate identified so far, expanding the known population and highlighting the diversity and potential formation mechanisms of diffuse radio sources.

Findings

J1248+4826 is the most compact ORC candidate with a radius of ~9" and extent of ~200 kpc.

The source's properties are consistent with known ORCs but its host galaxy's position is unusual.

The origin is likely fossil plasma re-accelerated by shocks, not active galactic nucleus activity.

Abstract

We report the discovery and investigate the nature of J1248+4826, an ORC-like source identified in the LOFAR Survey. We analyze its radio morphology, size, luminosity, and spectral properties, and study its environment and optical counterparts using multiwavelength data. We compare this source with other diffuse radio sources from the literature. J1248+4826 exhibits a well-defined ring of radius ~9" embedded in diffuse emission extending to ~1'. Assuming an association with a galaxy group at z=0.2, this corresponds to a physical radius of ~30 kpc, making it the most compact ORC candidate identified so far, while its total extent (~200 kpc), radio luminosity, and spectral index are consistent with the known ORC population. The putative host is the most massive group galaxy but it is located on the ring edge rather than in the center, unlike most known ORCs. We find no evidence for ongoing AGN activity, and both the morphology and spectral properties disfavor an origin as extended lobe from an active or dying radio galaxy, as well as fossil plasma re-accelerated by a virial shock. A more plausible scenario is that the source traces fossil plasma re-accelerated by shocks in the intragroup medium, possibly driven by galaxy interactions or mergers, although the compact ring size and high luminosity remain challenging to reproduce. If confirmed as an ORC, J1248+4826 would extend the population toward smaller physical scales and imply that compact systems may be underrepresented in current samples. The diffuse envelope further indicates that faint extended emission may be more common than previously recognized. This source highlights the diversity of diffuse radio sources and the likely role of group dynamics in shaping them, underscoring the need for larger samples, targeted follow-up observations and sophisticated simulations to discriminate between different formation scenarios.