The galaxy group merger origin of the Cloverleaf odd radio circle system

Odd radio circles (ORC) are a newly discovered class of extended faint radio sources of unknown origin. We report the first detection of diffuse X-ray gas at the location of a low-redshift ORC (z=0.046), known as Cloverleaf ORC. This observation was performed with the XMM-Newton X-ray telescope. The physical extent of the diffuse X-ray emission corresponds to approximately a 230 kpc by 160 kpc region, lying perpendicular to the radio emission detected by ASKAP. The X-ray spectrum shows characteristics of thermal multi-phase gas with temperatures of 1.10±0.08 keV and 0.22±0.01 keV and a central density of (4.9±0.6)×10⁻⁴ cm⁻³, indicating that the Cloverleaf ORC resides in a low-mass galaxy group. Using velocity dispersion measurements in the optical, the halo mass of the galaxy group has been reported as ~1.6×10¹³ Mₛᵤₙ. The presence of a high-velocity subgroup identified in optical data, the orientation of the BCG, the disturbed morphologies of galaxies towards the east of the Cloverleaf ORC, and the irregular morphology of the X-ray emission suggest that this system is undergoing a galaxy group merger. The radio power of the ORC could be explained by the shock reacceleration of fossil cosmic rays generated by a previous episode of black hole activity in the central active galactic nucleus.