In a recent paper we have shown that a minimally coupled , self-interacting scalar field of mass m can form black holes of mass M = \sqrt { 3 } / ( 4 m ) ( in Planck units ) .
If dark matter is composed by axions , they can form miniclusters that for QCD axions have masses below this value .
In this work it is shown that for a scenario in which the axion mass depends on the temperature as m \propto T ^ { -6 } , minicluster masses above 0.32 M _ { \odot } , corresponding to an axion mass of 3 \times 10 ^ { -10 } eV , exceed M and can collapse into black holes .
If a fraction of these black holes is in binary systems , gravitational waves emitted during the inspiral phase could be detected by advanced interferometers like LIGO or VIRGO and by the planned Einstein Telescope .
For a detection rate of one event per year , the lower limits on the binary fraction are 10 ^ { -4 } and 10 ^ { -6 } for LIGO and Einstein Telescope respectively .