Application of the adiabatic compression scenario to the radio relic in the galaxy cluster Abell 3411-3412

Abstract

Radio relics are non-thermal, steep-spectrum (α <−1) diffuse radio sources found in the peripheral regions of galaxy clusters. The emission is produced through synchrotron radiation as relativistic electrons (γ 1000) move in helical paths through the magnetic fields of the intracluster medium (ICM). These electrons have to be (re-)accelerated at the position where the emission is observed since the electron diffusion time over distances greater than 50 kpc from any compact source is longer than their radiative lifetime (∼0.1 Gyr). Because radio relics have been observed to trace shock fronts, they are widely considered to originate in intracluster shock waves. However, the most widely-used model for describing this (re-)acceleration process at shock fronts, the diffusive shock acceleration (DSA) model, has several challenges, including the fact that it is inefficient at low shock Mach numbers. In light of these challenges, it is worthwhile to consider alternative mechanisms. One possibility is the adiabatic compression by a shock wave of a residual fossil electron population which has been left over from a radio galaxy jet. This project adapts the adiabatic compression model to act on a general electron spectrum. The model is then applied to the relic hosted in the merging galaxy cluster Abell 3411-3412, where a radio bridge between the relic and a radio galaxy has been observed, to try to reproduce the spatial structure of the spectral index of the relic. The results show that the adiabatic compression model can repro-duce the observed spectral indices across the relic. Furthermore, the adiabatic compression model appears to be able to reproduce the observed spectra for a shock Mach number that is lower than the value predicted by the DSA-type modelling of this relic and is in accordance with the values derived from X-ray observations. The results also seem to suggest that other mechanisms, such as an expansion phase or post-shock turbulence, affect the spectrum behind the shock front