Joint Hα and X-Ray Observations of Massive X-Ray Binaries. II. The Be X-Ray Binary and Microquasar LS I +61 303

We present the results of an Hα monitoring campaign on the BeXRB and microquasar system LS I +61 303. We use radial velocity measurements of He I lines in our spectra to reevaluate the orbital elements and to better establish the time of periastron. We list equivalent widths and other parameters for the Hα emission line and discuss the orbital phase related variations observed. We call attention to a dramatic episode of emission weakening that occurred in less than 1 day that probably resulted from exposure to a transient source of ionizing radiation. We argue that the increase in Hα and X-ray emission following periastron probably results from the creation of an extended density wave in the disk created by tidal forces. We also discuss estimates of the size of the disk from the Hα equivalent width measurements, and we suggest that the disk radius from the average equivalent width corresponds to a resonant truncation radius of the disk while the maximum equivalent width corresponds to a radius limited by the separation of the stars at periastron. We note that a nearby faint companion is probably an unrelated foreground object.