Fermi LAT Observations of LS 5039
  The first results from observations of the high mass X-ray binary LS 5039
using the Fermi Gamma-ray Space Telescope data between 2008 August and 2009
June are presented. Our results indicate variability that is consistent with
the binary period, with the emission being modulated with a period of 3.903 +/-
0.005 days; the first detection of this modulation at GeV energies. The light
curve is characterized by a broad peak around superior conjunction in agreement
with inverse Compton scattering models. The spectrum is represented by a power
law with an exponential cutoff, yielding an overall flux (100 MeV - 300 GeV) of
4.9 +/- 0.5(stat) +/- 1.8(syst) x 10^-7 photon cm^-2 s^-1, with a cutoff at 2.1
+/- 0.3(stat) +/- 1.1(syst) GeV and photon index Gamma = 1.9 +/- 0.1(stat) +/-
0.3(syst). The spectrum is observed to vary with orbital phase, specifically
between inferior and superior conjunction. We suggest that the presence of a
cutoff in the spectrum may be indicative of magnetospheric emission similar to
the emission seen in many pulsars by Fermi.
