Coherent curvature emission and radio pulsars

Maser curvature emission in the presence of curvature drift is re-examined in detail. A physical interpretation of this mechanism is given. The maser emission is possible only when there is a beam-type distribution of outflowing electrons or positrons with Lorentz factors satisfying $\gamma \gtrsim 10^3$, which is determined by the geometry of the pulsar magnetosphere. It is shown that, for amplification to be effective in the pulsar magnetosphere, the density of the electrons or positrons in the beam must be close to a critical value beyond which the particle flow is no longer controlled by the magnetic field. It is also shown that, to obtain appreciable amplification in the range 2 to 200 MHz, the magnetic flux density in the source region must be $B \gt 10^8$ G. This seems to rule out the mechanism for millisecond pulsars.