New evidence for a cosmological distribution of stellar mass primordial black holes

In this paper we show that to explain the observed distribution of amplitudes in a large sample of quasar lightcurves, a significant contribution from microlensing is required. This implies the existence of a cosmologically distributed population of stellar mass compact bodies making up a large fraction of the dark matter. Our analysis is based on the lightcurves of a sample of over 1000 quasars, photometrically monitored over a period of 26 years. The intrinsic variations in quasar luminosity are derived from luminous quasars where the quasar accretion disc is too large to be microlensed by stellar mass bodies, and then synthetic lightcurves for the whole sample are constructed with the same statistical properties. We then run microlensing simulations for each quasar with convergence in compact bodies appropriate to the quasar redshift assuming a $\Lambda$CDM cosmology. The synthetic lightcurve is then superimposed on the amplification pattern to incorporate the effects of microlensing. The distribution of the resulting amplitudes can then be compared with observation, giving a very close match. This procedure does not involve optimising parameters or fitting to the data, as all inputs such as lens mass and quasar disc size come from independent observations in the literature. The overall conclusion of the paper is that to account for the distribution of quasar lightcurve amplitudes it is necessary to include the microlensing effects of a cosmologically distributed population of stellar mass compact bodies, most plausibly identified as stellar mass primordial black holes.