Strong field effects on pulsar arrival times: general orientations

TL;DR

This paper extends a formalism for analyzing strong gravitational effects on pulsar signals passing near black holes to more general geometries, enabling better interpretation of observational data.

Contribution

It generalizes previous models by incorporating arbitrary orientations of pulsar spin axes and beam directions, enhancing the analysis of pulsar-black hole systems.

Findings

Formalism applied to general geometries with arbitrary alignments.

Identification of observer-dependent 'keyhole' emission directions.

Analysis of beam energy distribution in complex configurations.

Abstract

A pulsar beam passing close to a black hole can provide a probe of very strong gravitational fields even if the pulsar itself is not in a strong field region. In the case that the spin of the hole can be ignored, we have previously shown that all strong field effects on the beam can be understood in terms of two "universal" functions, $F(\phi_{\rm in})$ and $T(\phi_{\rm in})$ of the angle of beam emission $\phi_{\rm in}$; these functions are universal in that they depend only on a single parameter, the pulsar/black hole distance from which the beam is emitted. Here we apply this formalism to general pulsar-hole-observer geometries, with arbitrary alignment of the pulsar spin axis and arbitrary pulsar beam direction and angular width. We show that the analysis of the observational problem has two distinct elements: (i) the computation of the location and trajectory of an observer-dependent "keyhole" direction of emission in which a signal can be received by the observer; (ii) the determination of an annulus that represents the set of directions containing beam energy. Examples of each are given along with an example of a specific observational scenario.