Abstract
Relativistic plasmas in strong electromagnetic fields exhibit distinct properties compared to classical plasmas. In astrophysical environments, such as neutron stars, white dwarfs, active galactic nuclei, and shocks, relativistic plasmas are pervasive and are expected to play a crucial role in the dynamics of these systems. Despite their significance, experimental and theoretical studies of these plasmas have been limited. Here, we present the first ab initio high-resolution kinetic simulations of relativistic plasmas undergoing synchrotron cooling in a highly magnetized medium. Our results demonstrate that these plasmas spontaneously generate coherent linearly polarized radiation in a wide range of parameters via the electron cyclotron maser instability, with radiative losses altering the saturation mechanism. Thus, the plasma continuously amplify coherent radiation for substantially longer durations of time. These findings highlight fundamental differences in the behavior of relativistic plasmas in strongly magnetized environments and align with astronomical phenomena, such as pulsar emission and fast radio bursts.