2023MNRAS.526.6214S

The origin of arcmin-sized odd radio circles (ORCs) found in modern all-sky radio surveys remains uncertain, with explanations ranging from starburst/active galactic nucleus-driven shocks to supernova remnants (SNRs) in the low-density ambient medium. Using well-calibrated radio light-curve models, we assess the possibility that ORCs are radio SNRs in low ambient densities. Our models imply that if ORCs 1-5 are SNRs, they must be within 200-350 kpc from the Sun, given their observed flux densities and sizes. To be evolving in the circumgalactic medium of the Milky Way, our models imply ORCs 1-5 to be ejecta-dominated SNRs within 50 kpc, evolving in ambient densities of (0.2-1.2) × 10^–3 cm^–3. However, this is statistically unlikely because ORCs 1-5 would have ages <640 yr, much smaller than their expected lifetimes of ≳10⁵ yr at these densities. Additionally, the low SN rate implies only a few SNRs within 50 kpc. On the other hand, the circumgalactic medium SNR scenario for J0624-6948 is more likely (although still low probability) compared to ORCs 1-5, as our models allow J0624-6948 to be ≲3000 yr. The interpretation of J0624-6948 as a Sedov-Taylor SNR in the Large Magellanic Cloud is also possible for a wide range of ambient densities (6 × 10^–4 to 0.5 cm^–3), consistent with the local H I environment, and ages ∼(0.2-2.6) × 10⁴ yr. Our work implies that while some ORCs may be SNRs, others are more likely large-scale shocks in distant galaxies.