Bayesian Periodic Signal Detection. II. Discovery of Periodic Phase Modulation in LS I +61o303 Radio Outbursts

A Bayesian analysis of the phase variations of the 26.5 day periodic radio outbursts from the high-mass X-ray binary LS I +61°303 demonstrates a clear periodic modulation on a timescale similar to that previously found for the long-term modulation of the outburst peak flux density. Combining the outburst phase and flux information we obtain a phase and flux modulation period of 1584+14-11 days as well as a more accurate outburst period of 26.4917 ± 0.0025 days. From the shape of the phase and outburst flux modulation we find that larger outbursts occur at an earlier orbital phase, closer to periastron, probably as a result of variations in the wind from the rapidly rotating Be star primary. The phase modulation also suggests a rather sudden onset to each new cycle of mass loss by the Be star. The next maximum in long-term flux modulation is predicted to occur between 1999 February and 2000 March (JD 2,451,233-2,451,633).