A Free-Electron Laser in the Pulsar Magnetosphere

We have studied systematically the free-electron laser in the context of high brightness pulsar radio emission. In this paper, we have numerically examined the case where a transverse electromagnetic wave is distorting the motion of a relativistic electron beam while travelling over one stellar radius (≈ 10 km). For different sets of parameters, coherent emission is generated by bunches of beam electrons in the radio domain, with bandwidths of 3 GHz. Pulse power often reached 10W, which corresponds with brightness temperature of 10K. The duration of these pulses is of the order of nanoseconds. In the context of pulsar radio emission, our results indicate that the laser can produce elementary bursts of radiation which build up the observed microstructures of a few tens of microseconds duration. The process is sensitive mostly to the beam particles energy, number density and the background magnetic field, but much less so to the transverse wave parameters. We demonstrate that the operation of a free-electron laser with a transverse electromagnetic wiggler in the pulsar magnetosphere occurs preferably at moderate Lorentz factors γ ≥ 100, high beam density n & 0.1 nGJ(r∗) where nGJ(r∗) is the Goldreich-Julian density at a stellar radius r∗, and finally, at large altitude where the background magnetic field is low B0 ≤ 10 −2 T.