Shocks and Radio Relics in Binary Cluster Mergers

Abstract

Shocks and Radio Relics in Binary Cluster Mergers

Hyesung Kang, Dongsu Ryu, Jeongbhin Seo

Galaxy cluster mergers drive large-scale shocks and turbulence that can accelerate cosmic-ray (CR) electrons and power diffuse radio relics. We use magnetohydrodynamic simulations of binary major mergers with mass ratios below three to investigate how merger dynamics, magnetic fields, and fossil plasma shape relic emission. The model includes diffusive shock acceleration as well as synchrotron, inverse-Compton, and Coulomb losses, while turbulent acceleration is represented through a simplified momentum-diffusion term. In our simulations, the equatorial shocks that form near pericenter are weak and inefficient, whereas the two axial shocks dominate particle acceleration and radio emission. The shock preceding the heavier subcluster (Shock 1) develops higher Mach numbers because it encounters cooler and faster infalling gas, even though it is physically smaller than Shock 2. Turbulent dynamos generate filamentary magnetic fields that strongly shape the brightness and fine-scale structure of radio relics. We further find that turbulent acceleration on timescales \tau_{pp}< 1~Gyr can delay postshock CR electron cooling and substantially brighten relics. Finally, fossil electron populations enhance emission without altering the spectral index distribution. Together, these results clarify the physical processes that govern the appearance of cluster radio relics.