Search

We present the serendipitous discovery of (1) a large double radio relic associated with the galaxy cluster PSZ2 G277.93 + 12.34 and (2) a new odd radio circle, ORC J1027–4422, both found in the same deep MeerKAT 1.3 GHz wide-band radio continuum image. The angular separation of the two arc-shaped cluster relics is ∼16 arcmin or ∼2.6 Mpc for a cluster redshift of z ≈ 0.158. The thin southern relic, which shows several ridges/shocks including one possibly moving inwards, has a linear extent of ∼1.64 Mpc. In contrast, the northern relic is about twice as wide, twice as bright, but only has a largest linear size of ∼0.66 Mpc. Complementary SRG/eROSITA X-ray images reveal extended emission from hot intracluster gas between the two relics and around the narrow-angle tail (NAT) radio galaxy PMN J1033–4335 (z ≈ 0.153) located just east of the northern relic. The radio morphologies of the NAT galaxy and the northern relic, which are also detected with the Australian Square Kilometer Array Pathfinder (ASKAP) at 888 MHz, suggest both are moving in the same outward direction. The discovery of ORC J1027–4422 in a different part of the same MeerKAT image makes it the fourth known single ORC. It has a diameter of ∼90 arcsec corresponding to 400 kpc at a tentative redshift of z ≈ 0.3 and remains undetected in X-ray emission. Supported by simulations, we discuss similarities between outward moving galaxy and cluster merger shocks as the formation mechanisms for ORCs and radio relics, respectively. [ABSTRACT FROM AUTHOR]

A joint hadronic model is shown to quantitatively explain the observations of diffuse radio emission from galaxy clusters in the form of minihaloes, giant haloes, relics, and their hybrid, transitional stages. Cosmic-ray diffusion of order D ∼ 1031–32 cm2 s−1, inferred independently from relic energies, the spatial variability of giant-halo spectra, and the spectral evolution of relics, reproduces the observed spatio-spectral distributions, explains the recently discovered mega-haloes as enhanced peripheral magnetization, and quenches electron (re)acceleration by weak shocks or turbulence. For instance, the hard-to-soft evolution along secondary-electron diffusion explains both the soft spectra in most halo peripheries and relic downstreams, and the hard spectra in most halo centres and relic edges, where the photon index can reach α ≃ −0.5 regardless of the Mach number |$\mathcal {M}$| of the coincident shock. Such spatio-spectral modelling, recent γ-ray observations, and additional accumulated evidence are thus shown to support a 2010 claim that the seamless transitions among minihaloes, giant haloes, and relics, their similar energetics, integrated spectra, and delineating discontinuities, the inconsistent |$\mathcal {M}$| inferred from radio versus X-rays in leptonic models, and additional observations, all indicate that these diffuse radio phenomena are manifestations of the same cosmic-ray ion population, with no need to invoke less natural alternatives. [ABSTRACT FROM AUTHOR]