  The process of superradiance can extract angular momentum and energy from
astrophysical black holes (BHs) to populate gravitationally-bound states with
an exponentially large number of light bosons. We analytically calculate
superradiant growth rates for vectors around rotating BHs in the regime where
the vector Compton wavelength is much larger than the BH size. Spin-1 bound
states have superradiance times as short as a second around stellar BHs,
growing up to a thou- sand times faster than their spin-0 counterparts. The
fast rates allow us to use measurements of rapidly spinning BHs in X-ray
binaries to exclude a wide range of masses for weakly-coupled spin-1 particles,
$5 \times 10^{-14} - 2 \times 10^{-11}$ eV; lighter masses in the range $6
\times 10^{-20} - 2 \times 10^{-17}$ eV start to be constrained by supermassive
BH spin measurements at a lower level of confidence. We also explore routes to
detection of new vector particles possible with the advent of gravitational
wave (GW) astronomy. The LIGO-Virgo collaboration could discover hints of a new
light vector particle in statistical analyses of masses and spins of merging
BHs. Vector annihilations source continuous monochromatic gravitational
radiation which could be observed by current GW observatories. At design
sensitivity, Advanced LIGO may measure up to thousands of annihilation signals
from within the Milky Way, while hundreds of BHs born in binary mergers across
the observable universe may superradiate vector bound states and become new
beacons of monochromatic gravitational waves.
