<i>FERMI</i> LAT OBSERVATIONS OF LS I +61°303: FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS

Abstract

This Letter presents the first results from the observations of LSI +61 303\nusing Large Area Telescope data from the Fermi Gamma-Ray Space Telescope\nbetween 2008 August and 2009 March. Our results indicate variability that is\nconsistent with the binary period, with the emission being modulated at 26.6\n+/- 0.5 days. This constitutes the first detection of orbital periodicity in\nhigh-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized\nby a broad peak after periastron, as well as a smaller peak just before\napastron. The spectrum is best represented by a power law with an exponential\ncutoff, yielding an overall flux above 100 MeV of 0.82 +/- 0.03(stat) +/-\n0.07(syst) 10^{-6} ph cm^{-2} s^{-1}, with a cutoff at 6.3 +/- 1.1(stat) +/-\n0.4(syst) GeV and photon index Gamma = 2.21 +/- 0.04(stat) +/- 0.06(syst).\nThere is no significant spectral change with orbital phase. The phase of\nmaximum emission, close to periastron, hints at inverse Compton scattering as\nthe main radiation mechanism. However, previous very high-energy gamma ray\n(&gt;100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to\napastron. This and the energy cutoff seen with Fermi suggest the link between\nHE and VHE gamma rays is nontrivial.\n