ICM-SHOX III. The case of MACS J0018.5+1626, a radio relic that looks like a radio halo?

TL;DR

This paper tackles the origin of diffuse radio emission in MACS J0018.5+1626 and tests whether standard DSA can account for LOFAR 144 MHz observations given a binary merger scenario. It uses a hybrid framework that couples 3D MHD simulations with a Fokker-Planck solver for cosmic-ray transport, constrained by the ICM-SHOX pipeline to match multi-probe observables. The results indicate two merger-driven shocks with typical Mach numbers $\mathcal{M}_s \sim 2$--$3$ and $\sigma_{\mathcal{M}} \sim 0.5$--$1.5$, consistent with an interpretation that the LOFAR emission arises from the superposition of two relics viewed nearly face-on. The approach demonstrates the power of combining multi-wavelength data with forward modeling to reconstruct merger histories and interpret non-thermal ICM emission.

Abstract

We present the first detailed numerical modeling of the radio emission from MACS J0018.5+1626 as part of the Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX) project. By matching X-ray, thermal and kinetic Sunyaev--Zel'dovich, optical and lensing observables to simulations, the ICM-SHOX pipeline indicates that MACS J0018.5+1626 is undergoing a binary merger close to pericenter passage and is observed along a line of sight nearly aligned with the merger axis. We perform three-dimensional magnetohydrodynamic simulations of binary cluster mergers coupled to tracer particles and a Fokker--Planck solver to model the radio emission. Exploring variations in the most likely initial conditions within the ICM-SHOX parameter space, such as the relative cluster velocity and impact parameter, we find that the resulting merger configuration consistently produces two merger-driven shocks with typical average Mach numbers $\mathcal{M}_s \sim 2$--$3$ with corresponding standard deviations of $σ_{\mathcal{M}} \sim 0.5$--$1.5$. Within this framework, we examine the cluster conditions under which standard diffusive shock acceleration can reproduce LOFAR observations. In particular, we discuss the possibility that the apparent radio halo seen by LOFAR arises from the superposition of two radio relics viewed nearly face-on.