  We report the results from our analysis of 8 years of the data for the
gamma-ray binary LS I +61 303, obtained with the Large Area Telescope onboard
the Fermi Gamma-Ray Space Telescope. We find a significant dip around the
binary's periastron in the superorbital light curves, and by fitting the light
curves with a sinusoidal function, clear phase shifts are obtained. The
superorbital modulation seen in the binary has been long known and different
scenarios have been proposed. Based on our results, we suggest that the
circumstellar disk around the Be companion of this binary has an elliptical
shape and the major axis of the disk rotates at the superorbital period of 1667
days. As a result, the density of the ambient material around the compact star
of the binary changes along the binary orbit over the superorbital period,
causing the phase shifts in the modulation, and around periastron, the compact
star probably enters the disk, causing the appearance of the dip. Numerical
simulations may be conducted in order to study the detailed physical processes
and verify our suggestion.
