Author(s): A. Abdo A., Ballet J., Bruel P., Casandjian J.M., Chaty S., Cohen-Tanugi J., Dubus G., Dumora D., Farnier C., Fegan S.J., Fortin P., Giebels B., Grenier I.A., Grondin M.H., Guillemot L., Guiriec Sylvain, Hill A.B., Horan D., Knödlseder J., Lemoine-Goumard M., Lott B., Nuss E., Parent D., Pelassa V., Piron F., Reposeur T., Sanchez D., Smith D. A., Tibaldo L., Vilchez N.
(Article) Published:
The Astrophysical Journal / The Astrophysical Journal Letters, vol. 706 p.L56-L61 (2009)
Links openAccess full text : 
Ref HAL: in2p3-00436279_v1
Ref Arxiv: 0910.5520
DOI: 10.1088/0004-637X/706/1/L56
Ref. & Cit.: NASA ADS
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102 citations
Abstract:
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.
Comments: 6 pages, 5 figures, accepted for publication in ApJ Letters. Contact authors: Adam B. Hill (adam.hill@obs.ujf-grenoble.fr), Richard Dubois (richard@slac.stanford.edu), Takaaki Tanaka (ttanaka@slac.stanford.edu), Robin Corbet (Robin.Corbet@nasa.gov)