D. Hadasch, for the Fermi-LAT collaboration
The Fermi Large Area Telescope (LAT) has made the first definitive GeV
detections of the binaries LS I +61^{\circ}303 and LS5039 in the first year
after its launch in August 2008. These detections were unambiguous because,
apart from a reduced positional uncertainty, the gamma-ray emission in each
case was orbitally modulated with the corresponding orbital period. The LAT
results posed new questions about the nature of these objects, after the
unexpected observation of an exponential cutoff in the GeV gamma-ray spectra of
both LS I +61^{\circ}303 and LS5039, at least along part of their orbital
motion. We present here the analysis of new data from the LAT, comprising 2.5
years of observations through which LS I +61^{\circ}303 continues to provide
some surprises. We find an increase in flux in March 2009 and a steady decrease
in the flux fraction modulation. The LAT now detects emission up to 30 GeV,
where prior datasets led to upper limits only. At the same time,
contemporaneous TeV observations either no longer detected the source, or found
it -at least in some orbits- close to periastron, far from the usual phases in
which the source usually appeared at TeV energies. The on-source exposure of LS
5039 has also drastically increased along the last years, and whilst our
analysis shows no new behavior in comparison with our earlier report, the
higher statistics of the current dataset allows for a deeper investigation of
its orbital and spectral evolution.
View original:
http://arxiv.org/abs/1111.0350
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