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.
