M. R. S. Hawkins

The possibility that stellar mass primordial black holes may make up at least a significant fraction of dark matter has recently received much attention, partly as a result of gravitational wave observations, but more specifically from observations of microlensing in the Galactic halo and in quasar gravitational lens systems. If this is the case then a number of observable consequences are to be expected. This paper focusses on the prediction that dark matter in the form of primordial black holes will result in a web of caustics which when traversed by quasars will result in a complex but characteristic amplification of the accretion disc light source. Caustic crossings produce features in quasar light curves which are relatively straightforward to identify, and are hard to associate with any intrinsic mode of variation. Microlensing simulations are used to clarify the nature of the expected light curve features, and compared with observed light curves to demonstrate that caustic crossing features can be present. A further test of microlensing is based on the expected statistical symmetry of the light curves, which is not predicted for most models of intrinsic quasar variability, but is found in large samples of quasar light curves. The conclusion of the paper is that observations of quasar light curves are consistent with the expected microlensing amplifications from dark matter made up of stellar mass primordial black holes, but cannot easily be explained by intrinsic variations of the quasar accretion disc.