Black holes in binaries with other compact objects can provide natural venues for indirect detection of axions or other ultralight fields .
The superradiant instability associated with a rapidly spinning black hole leads to the creation of an axion cloud which carries energy and angular momentum from the black hole .
This cloud will then decay via gravitational wave emission .
We show that the energy lost as a result of this process tends toward an outspiraling of the binary orbit .
A given binary system is sensitive to a narrow range of axion masses , determined by the mass of the black hole .
Pulsar-black hole binaries , once detected in the electromagnetic band , will allow high-precision measurements of black hole mass loss via timing measurements of the companion pulsar .
This avenue of investigation is particularly promising in light of the recent preliminary announcements of two candidate black hole-neutron star mergers by LIGO/VIRGO ( # S190814bv and # S190426c ) .
We demonstrate that for such a binary system with a typical millisecond pulsar and a 3 M _ { \odot } black hole , axions with masses between 2.7 \times 10 ^ { -12 } eV and 3.2 \times 10 ^ { -12 } eV are detectable .
Recent gravitational wave observations by LIGO/VIRGO of binary black hole mergers imply that , for these binaries , gravitational radiation from the rotating quadrupole moment is a dominant effect , causing an inspiraling orbit .
With some reasonable assumptions about the period of the binary when it formed and the spins of the black holes , these observations rule out possible axion masses between 3 \times 10 ^ { -13 } eV and 6 \times 10 ^ { -13 } eV .
Future binary black hole observations , for example by LISA , are expected to provide more robust bounds .
In some circumstances , neutron stars may also undergo superradiant instabilities , and binary pulsars could be used to exclude axions with certain masses and matter couplings .