Synchrotron Firehose Instability

TL;DR

This paper introduces the synchrotron firehose instability (SFHI), a process where pressure anisotropy in a cooling plasma leads to magnetic fluctuations that regulate particle distributions, relevant in astrophysical jets and accretion flows.

Contribution

It demonstrates, through linear theory and PIC simulations, the existence and nonlinear behavior of SFHI in relativistic plasmas, including electron-positron and electron-ion cases, and explores its astrophysical implications.

Findings

SFHI causes magnetic fluctuations that pitch-angle scatter particles.

Nonlinear cyclic evolution of firehose bursts observed in simulations.

Relativistic ions gain pressure anisotropy and cool via secondary instabilities.

Abstract

We demonstrate using linear theory and particle-in-cell (PIC) simulations that a synchrotron-cooling collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability, in a process that we dub the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into small-amplitude, kinetic-scale magnetic-field fluctuations, which pitch-angle scatter the particles and bring the plasma to a near-thermal state of marginal instability. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. We compare the SFHI for electron-positron and electron-ion plasmas. As a byproduct of the growing electron-firehose magnetic field fluctuations, magnetized ions gain a pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they also experience cooling due to collisionless thermal coupling with the electrons, which we argue is mediated by a secondary ion-cyclotron instability. We suggest that the SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black-hole accretion flows and relativistic jets, where the redistribution of energetic electrons from low to high pitch angles may cause transient bursts of radiation.