Impact of Postshock Turbulence on the Radio Spectrum of Radio Relic Shocks in Merging Clusters

TL;DR

This paper models how postshock magnetic turbulence and its decay influence the radio emission from relics in merging galaxy clusters, emphasizing the importance of turbulence in CR electron aging and spectral features.

Contribution

It introduces analytic models of magnetic turbulence decay and CR acceleration to better interpret radio relic observations in galaxy clusters.

Findings

Turbulent acceleration delays CR electron aging.

Magnetic field decay reduces synchrotron emission.

Spectral index-based Mach numbers are overestimated.

Abstract

This study investigates the impact of magnetic turbulence on cosmic ray (CR) electrons through Fermi-II acceleration behind merger-driven shocks in the intracluster medium and examines how the ensuing synchrotron radio emission is influenced by the decay of magnetic energy through dissipation in the postshock region. We adopt simplified models for the momentum diffusion coefficient, specifically considering transit-time-damping resonance with fast-mode waves and gyroresonance with Alfv\'en waves. Utilizing analytic solutions derived from diffusive shock acceleration theory, at the shock location, we introduce a CR spectrum that is either shock-injected or shock-reaccelerated. We then track its temporal evolution along the Lagrangian fluid element in the time domain. The resulting CR spectra are mapped onto a spherical shell configuration to estimate the surface brightness profile of the model radio relics. Turbulent acceleration proves to be a significant factor in delaying the aging of postshock CR electrons, while decaying magnetic fields have marginal impacts due to the dominance of inverse Compton cooling over synchrotron cooling. However, the decay of magnetic fields substantially reduces synchrotron radiation. Consequently, the spatial distribution of the postshock magnetic fields affects the volume-integrated radio spectrum and its spectral index. We demonstrate that the Mach numbers estimated from the integrated spectral index tend to be higher than the actual shock Mach numbers, highlighting the necessity for accurate modeling of postshock magnetic turbulence in interpreting observations of radio relics.