Synchrotron Firehose Instability

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