General relativistic effects on the pulse profile of fast pulsars

Abstract

The recent discovery of millisecond pulsars has led to several theories about the structure and radiation characteristics of rapidly rotating neutron stars. The rapid rotation rate implies that such pulsars will be compact (implying large space-time curvature) and that surface emission characteristics will differ significantly from the non-rotational (Schwarzchild) case. We report here the implications of large space-time curvature and large rotation (specifically, the dragging of inertial frames introduced by rotation) on the trajectories of photons, and hence, on the pulse profile of fast pulsars. Space-time curvature leads to substantial amounts of divergence in the pulse width and reduction in the pulse intensity whereas rotation produces a tilt of the pulse cone from its original direction of emission, and deforms the cone, introducing an asymmetry in the (flattened) pulse profile leading to a time delay in the arrival of photons emitted within the pulse cone. We find that the effect of curvature dominates over that of rotation, suggesting a larger brightness temperature than inferred observationally for all pulsars whatever the rotation rate (unless the pulsar is of very low mass).