Indexed on: 03 Nov '98Published on: 03 Nov '98Published in: Astrophysics
For radio pulsars in orbit with a compact companion, pulsar timing observations have proved to be a powerful tool for identifying the physical nature of the companion. Unfortunately, perhaps the most intriguing system where such a tool could be used, a pulsar in orbit with a black hole, has yet to be discovered. In this paper we give a detailed investigation of what one can learn about the black hole companion via timing observations of the pulsar. We present an analytic calculation for the propagation delay caused by the frame-dragging effect and show that it has the same functional behavior as the modulation of the observed rotational phase of the pulsar caused by the deflection of the radio signals in the gravitational field of the companion (bending delay). Thus, contrary to statements of other authors, the frame-dragging delay is unlikely to be separately measurable in pulsar binaries where the companion is a stellar mass black hole. We demonstrate, however, that the precession of the binary orbit caused by the relativistic spin-orbit coupling can lead to observable effects which can be used to set a lower limit to the black-hole spin, or possibly allow the determination of its magnitude and orientation. We give parameter estimates for two possible systems, a 10 solar mass black hole in orbit either with a young (~0.1 s) pulsar or with a millisecond pulsar. Finally, we discuss the measurability of the quadrupole moment of the rotating black hole companion which would test the presence of a Kerr black hole. As an interesting side result of our calculations, we can give a further argument why the companion of PSR J0045-7319 cannot be a Kerr black hole.