Gravitational lensing of pulsars as a probe of dark matter halos

Abstract

A particular open problem in cosmology is whether dark matter on small scales is clumpy, forming gravitationally-bound halos distributed within the Galaxy. The practical difficulties inherent in testing this hypothesis stem from the fact that, on astrophysical scales, dark matter is solely observable via its gravitational interaction with other objects. This thesis presents a gravitational-lensing-based solution for the mapping and characterisation of low-mass, dark matter halos via their signature in millisecond pulsar observations. This involves: first, determining the time delay and magnification surfaces generated in the frame of reference of the halo; second, obtaining the corresponding pulsar signature in the reference frame of the observer; and last, generalising the method to multiple halos at varying distances. We discuss whether the delay is observationally detectable for both single and multiple lenses. The key dependency of the time delay is the density profile adopted for the halo. I utilise a variety of proposed halo mass profiles -- elliptical, Schwarzschild, horizontal-disc lenses and the Navarro-Frenk-White (NFW) density profile -- which are applicable over a broad range of halo masses. I demonstrate the use of Hankel transforms to increase the efficiency of the relativistic time delay calculation. The observational signatures of such halos are best identified using millisecond pulsars due to their high rotational frequencies and period stability. My method does not require major adjustments when searching for signs of lensing, thus it is unnecessary to implement specialist data reduction pipelines. Thus we can leverage data from both existing and future surveys easily. This method is readily extensible to nearby globular clusters and galaxies, pending improvements in pulsar detection at such distances.