Synchrotron masers and fast radio bursts

Fast radio bursts, with a typical duration of 1 ms and 1 Jy flux density at gigahertz frequencies, have brightness temperatures exceeding 10(33) K, requiring a coherent emission process. This can be achieved by bunching particles in volumes smaller than the typical wavelength, but this may be challenging. Maser emission is a possibility. Under certain conditions, the synchrotron-stimulated emission process can be more important than true absorption, and a synchrotron maser can be created. This occurs when the emitting electrons have a very narrow distribution of pitch angles and energies. This process overcomes the difficulties of having extremely dense bunches of particles and relaxes the light-crossing time limits, since there is no simple relation between the actual size of the source and the observed variability time-scale.