$\mathcal O(10) M_\odot$ primordial black holes and string axion dark
  matter
  LIGO-Virgo collaboration has found black holes as heavy as $M \sim 30M_\odot$
through the detections of the gravitational waves emitted during their mergers.
Primordial black holes (PBHs) produced by inflation could be an origin of such
events. While it is tempting to presume that these PBHs constitute all Dark
Matter (DM), there exists a number of constraints for PBHs with $\mathcal{O}
(10) M_\odot$ which contradict with the idea of PBHs as all DM. Also, it is
known that weakly interacting massive particle (WIMP) that is a common DM
candidate is almost impossible to coexist with PBHs. These observations
motivate us to pursue another candidate of DM. In this paper, we assume that
the string axion solving the strong CP problem makes up all DM, and discuss the
coexistence of string axion DM and inflationary PBHs for LIGO events.
