On the effect of the light bending phenomenon for a pulsar in a binary with a Kerr black hole

TL;DR

This study analyzes how light bending caused by Kerr black holes affects pulsar signals in binary systems, revealing minimal influence from black hole spin on delays but significant effects near conjunctions and with super-massive black holes.

Contribution

It provides a detailed comparison of light-bending effects on pulsar signals for rotating black holes versus non-rotating ones, including the impact of spin and mass scale.

Findings

Spin introduces nanosecond order changes in delays.

Discontinuities occur at orbital phases with light path changes.

Bending delays are much larger for super-massive black holes.

Abstract

We study the effect of light-bending on the signal of a pulsar in binaries with rotating black hole companions, focusing on stellar mass black holes. We show that the impacts of various parameters on the bending delays visually match with those observed for a non-rotating black holes, because the magnitude of the spin as well as the orientation of the spin axis of the black hole introduce changes in the nanosecond order and other parameters do so in the microsecond order. Consequently, the distortion of the beam and the resulting changes in the pulse shape are minimally influenced by spin-related parameters of the black hole. We also investigate the impact of various parameters on the difference of the delays with and without the spin of the black hole and notice nanosecond scale discontinuities at orbital phases where the path of the light ray changes its direction with respect to the direction of the spin of the black hole. Moreover, as in the Schwarzschild case, the bending delays become irregular (on the microsecond scale) near the superior conjunction. We also explore the effect of bending on the pulse profiles and bending delays if the companion of the pulsar is a rotating super-massive black hole. We find significant enhancement and change in the shape of the profiles at and near the superior conjunction in comparison to stellar mass black holes. Moreover, bending delays are about three orders of magnitude higher than those in case of the stellar mass black holes.