Strong field effects on pulsar arrival times: circular orbits and equatorial beams

TL;DR

This paper develops a simple formalism to analyze strong gravitational effects on pulsar signals near black holes, revealing secondary pulses and phase relationships as probes of spacetime curvature.

Contribution

Introduces a universal function-based formalism to compute pulse timing and intensity near black holes, including secondary pulses, for pulsars in circular orbits.

Findings

Secondary pulses can be as bright as primary ones under certain conditions.

Phase relationships between pulses reveal spacetime geometry.

The formalism applies to non-rotating black holes and generalizes to non-planar pulsar orbits.

Abstract

If a pulsar orbits a supermassive black hole, the timing of pulses that pass close to the hole will show a variety of strong field effects. To compute the intensity and timing of pulses that have passed close to a nonrotating black hole we introduce here a simple formalism based on two "universal functions," one for the bending of photon trajectories and the other for the photon travel time on these trajectories. We apply this simple formalism to the case of a pulsar in circular orbit that beams its pulses into the orbital plane. In addition to the "primary" pulses that reach the receiver by a more-or-less direct path, we find that there are secondary and higher order pulses. These are usually much dimmer than the primary pulses, but they can be of comparable or even greater intensity if they are emitted when pulsar is on the side of the hole furthest from the receiver. We show that there is a phase relationship of the primary and secondary pulses that is a probe of the strongly curved spacetime geometry. Analogs of these phenomena are expected in more general configurations, in which a pulsar in orbit around a hole emits pulses that are not confined to the orbital plane.