DSA efficacy at weak intracluster shocks (including fossil electrons)

Determine whether diffusive shock acceleration can efficiently accelerate electrons at weak intracluster medium shocks and, in particular, establish whether fossil electron populations enable electron acceleration under these subcritical conditions.

Background

In the simulations, merger-driven shocks that intersect the jet-injected cosmic-ray material are generally weak (typical Mach numbers ≲2), especially at late times. Particle-in-cell and hybrid-kinetic studies suggest a critical Mach number below which standard electron DSA is inefficient.

The authors highlight that it remains unresolved whether DSA can operate at such weak shocks and specifically whether pre-energized (fossil) electrons change this conclusion. This uncertainty directly impacts interpretations of radio relics formed by re-acceleration at cluster shocks.