Broad-band non-thermal emission of odd radio circles induced by explosive galactic outflow remnants and their evolution

Abstract

Odd radio circles (ORCs) are mysterious rings of faint, diffuse emission recently discovered in radio surveys, some of which may be associated with galaxies in relatively dense environments. We propose that such ORCs are synchrotron emission from remnants of explosive galactic outflows, calling them OGREs, and discuss their broad-band non-thermal emission and evolution. We posit that a large amount of energy was ejected from the central galaxy in the past, creating an outgoing shock that accelerates cosmic rays. Assuming plausible values for the density, temperature and magnetic field of the ambient medium, consistency with the observed spectral index, size, and power of the ORCs requires the energy to be as high as ∼1060 erg, suggesting that their sources could be active galactic nuclei. We calculate the spectral energy distributions (SEDs) of the OGREs and their evolution, including synchrotron, inverse Compton (IC), and bremsstrahlung emission from electrons, and pion-decay emission from protons. We find that the SEDs of the younger OGREs are not greatly different from those of older ones currently observable as ORCs if radiative cooling of electrons is effective. As such younger OGREs are expected to be rarer and smaller, they may not be readily observable. However, if radiative cooling of electrons is ineffective, younger OGREs may be detectable in X-rays.