Uniformity of radio illumination under pre-existing plasma re-acceleration at merger shocks

Determine whether re-acceleration of pre-existing plasma in the intracluster medium affects all merger-driven shock fronts in galaxy clusters in a similar manner, thereby producing uniform radio illumination across shock surfaces; establishing this is essential to assess if re-acceleration can consistently account for observed radio relic luminosities without invoking unphysically high electron acceleration efficiencies in diffusive shock acceleration models.

Background

Radio relics are thought to result from synchrotron emission by cosmic-ray electrons accelerated at cluster merger shocks via diffusive shock acceleration (DSA). However, reproducing observed relic luminosities under standard DSA can require very high, potentially nonphysical, electron acceleration efficiencies.

A commonly proposed resolution is the (re-)acceleration of a pre-existing population of relativistic plasma (e.g., fossil electrons). The extent to which this mechanism operates uniformly across different merger shocks is not established, and this uncertainty limits the ability to model radio relic luminosities and their spatial uniformity consistently across clusters.