2002ApJ...575..427G

The binary star LS I +61°303 is remarkable for its periodic radio outbursts every 26.5 days. We recently discovered a ∼4.4 yr periodic modulation of the phase and amplitude of these outbursts using the Gregory-Loredo Bayesian algorithm for detecting periodic signals of unknown shape. In this paper we obtain improved estimates of both the orbital period P₁ and the modulation period P₂ based on a larger data set and examine the behavior of the spectral index between 2.2 and 8.3 GHz versus the modulation period. The new estimates are P₁=26.4960±0.0028 and P₂=1667±8 days, and our best estimate of the radio phase of periastron is ∼0.4. Analysis of the spectral index data indicates that the optical depth in the synchrotron emission region is always ≪1 at 8.3 GHz and can reach values of ∼2.7 at 2.2 GHz. A test of the precessing Be star model of Lipunov & Nazin indicates that it is unlikely to be the correct mechanism to explain the 1667 day periodic modulation.