A New TeV Binary: The Discovery of an Orbital Period in HESS J0632+057
  HESS J0632+057 is a variable, point-like source of Very High Energy ($>100$
GeV) gamma-rays located in the Galactic plane. It is positionally coincident
with a Be star, it is a variable radio and X-ray source, has a hard X-ray
spectrum, and has low radio flux. These properties suggest that the object may
be a member of the rare class of TeV/X-ray binary systems. The definitive
confirmation of this would be the detection of a periodic orbital modulation of
the flux at any wavelength. We have obtained {\it Swift} X-ray telescope
observations of the source from MJD 54857 to 55647 (Jan. 2009 - Mar. 2011) to
test the hypothesis that HESS J0632+057 is an X-ray/TeV binary. We show that
these data exhibit flux modulation with a period of $321 \pm 5$ days and we
evaluate the significance of this period by calculating the null hypothesis
probability, allowing for stochastic flaring. This periodicity establishes the
binary nature of HESS J0632+057.
