Fermi LAT Observations of LS 5039

Understand

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).