Abdo, A. A.A. A.AbdoAckermann, M.M.AckermannAjello, M.M.AjelloCavazzuti, ElisabettaElisabettaCavazzutiCutini, SaraSaraCutiniGasparrini, DarioDarioGasparrini2020-09-172020-09-172009-11-01https://hdl.handle.net/20.500.13025/1360The Astrophysical Journal Letters, Volume 706, Issue 1, pp. L56-L61 (2009).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) 10<SUP>-7</SUP> photon cm<SUP>-2</SUP> s<SUP>-1</SUP>, 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.binaries closegamma rays observationsstars variables otherX-rays binariesX-rays individual LS 503910.1088/0004-637X/706/1/L56http://adsabs.harvard.edu/abs/2009ApJ...706L..56A2009ApJ...706L..56A