Synchrotron Firehose Instability

We demonstrate using linear theory and particle-in-cell (PIC) simulations that a synchrotron-cooling, relativistic collisionless plasma acquires pressure anisotropy and, if the plasma beta is sufficiently high, becomes unstable to the firehose instability. We call this process the synchrotron firehose instability (SFHI). The SFHI channels free energy from the pressure anisotropy of the radiating, relativistic electrons (and/or positrons) into kinetic-scale magnetic-field fluctuations, which bring the plasma to marginal stability through pitch-angle scattering. The PIC simulations reveal a nonlinear cyclic evolution of firehose bursts interspersed by periods of stable cooling. In an electron-ion plasma, the growing electron-firehose magnetic field fluctuations lead to an ion pressure anisotropy opposite to that of the electrons. If these ions are relativistically hot, we find that they experience cooling due to collisionless thermal coupling with the electrons, mediated by a secondary ion-cyclotron instability. The SFHI may be activated in a number of astrophysical scenarios, such as within ejecta from black-hole accretion flows and relativistic jets, where it may cause transient bursts of radiation.