Stochastic particle acceleration and the problem of backgroud plasma overheating

TL;DR

This paper investigates stochastic particle acceleration in galaxy clusters, demonstrating conditions under which plasma heating is minimized or even reversed, allowing for sustained non-thermal particle populations without overheating the plasma.

Contribution

It reveals that specific acceleration regimes can produce long-lived non-thermal tails while keeping plasma temperature stable or decreasing, challenging previous assumptions about Fermi acceleration inefficiency.

Findings

Plasma heating rate can be low or negative in certain acceleration regimes.

Non-thermal particle tails can persist for long times without overheating.

External acceleration can cool the plasma by removing high-energy particles.

Abstract

The origin of hard X-ray (HXR) excess emission from clusters of galaxies is still an enigma, whose nature is debated. One of the possible mechanism to produce this emission is the bremsstrahlung model. However, previous analytical and numerical calculations showed that in this case the intracluster plasma had to be overheated very fast because suprathermal electrons emitting the HXR excess lose their energy mainly by Coulomb losses, i.e., they heat the background plasma. It was concluded also from these investigations that it is problematic to produce emitting electrons from a background plasma by stochastic (Fermi) acceleration because the energy supplied by external sources in the form of Fermi acceleration is quickly absorbed by the background plasma. In other words the Fermi acceleration is ineffective for particle acceleration. We revisited this problem and found that at some parameter of acceleration the rate of plasma heating is rather low and the acceleration tails of non-thermal particles can be generated and exist for a long time while the plasma temperature is almost constant. We showed also that for some regime of acceleration the plasma cools down instead of being heated up, even though external sources (in the form of external acceleration) supply energy to the system. The reason is that the acceleration withdraws effectively high energy particles from the thermal pool (analogue of Maxwell demon).