  X-ray binaries are composed of a normal star in orbit around a neutron star
or stellar-mass black hole. Radio and X-ray observations have led to the
presumption that some X-ray binaries called microquasars behave as scaled down
active galactic nuclei. Microquasars have resolved radio emission that is
thought to arise from a relativistic outflow akin to active galactic nuclei
jets, in which particles can be accelerated to large energies. Very high energy
gamma-rays produced by the interactions of these particles have been observed
from several active galactic nuclei. Using the High Energy Stereoscopic System,
we find evidence for gamma-ray emission >100 GeV from a candidate microquasar,
LS 5039, showing that particles are also accelerated to very high energies in
these systems.
