General relativistic effects on the pulse profile of fast pulsars

Abstract

The recent discovery of millisecond pulsars^1,2 has led to several theories about the structure^3–8 and radiation characteristics^9–11 of rapidly rotating neutron stars. The rapid rotation rate implies that such pulsars will be compact⁵ (implying large space-time curvature) and that surface emission characteristics will differ significantly from the non-rotational (Schwarzchild) case¹¹. 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).