Effect of clustering on primordial black hole microlensing constraints
  Stellar microlensing observations tightly constrain compact object dark
matter in the mass range $(10^{-11} - 10^{3}) M_{\odot}$. Primordial Black
Holes (PBHs) form clusters, and it has been argued that these microlensing
constraints are consequently weakened or evaded. For the most commonly studied
PBH formation mechanism, the collapse of large gaussian curvature perturbations
generated by inflation, the clusters are sufficiently extended that the PBHs
within them act as individual lenses. We find that if the typical mass of the
clusters is sufficiently large, $ \gtrsim 10^{6} M_{\odot}$, then the event
duration distribution can deviate significantly from that produced by a smooth
dark matter distribution, in particular at the shortest durations. As a
consequence of this, the probability distribution of the number of observed
events is non-Poissonian, peaking at a lower value, with an extended tail to
large numbers of events. However, for PBHs formed from the collapse of large
inflationary perturbations, the typical cluster is expected to contain $\sim
10^{3}$ PBHs. In this case the effect of clustering is negligibly small, apart
from for the most massive PBHs probed by decade-long stellar microlensing
surveys ($M_{\rm PBH} \sim 10^{3} M_{\odot}$).
