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

TL;DR

This paper models the radio emission of MACS J0018.5+1626 using numerical simulations, suggesting the observed radio halo may be a superposition of two relics from a binary cluster merger.

Contribution

It provides the first detailed numerical modeling of radio emission for this cluster, linking merger dynamics to radio features and proposing a new interpretation of the radio halo.

Findings

Merger-driven shocks with Mach numbers ~2-3 are produced in simulations.

The radio halo may be a superposition of two relics viewed face-on.

Simulations support diffusive shock acceleration as a mechanism for radio 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 $\sigma_{\mathcal{M}} \sim 0.5$--$1.5$. 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.