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Long-term optical spectral observations of the Be / X-ray binary LSI + 61 ◦ 303 : evidence of H α emission associated with the neutron star

We present the optical spectra of the periodic radio star LSI +61◦303 with intermediate dispersion. The H α emission line profile and its variation are analyzed. Variability in LSI +61◦303 is observed on different timescales. During our observations the equivalent width (EW) of H α line showed long-term variability that is likely to be associated with the 4-year period. We suggest that the long-term variability is connected to the intrinsic behaviour of the Be star. The Hα EW changed strongly around the radio maximum, clearly displayed a maximum at phases between 0.55–0.6, and possibly both another maximum at phases around 1.0 and a minimum between radio phases 0.7–0.9. The only three spectra that showed nearly equal intensities of the two peaks of H α emission line all occurred at radio phases around 0.8. It is interesting to note that the Hα emission line in 1995 October showed unusual variation at radio phases around 0.6, where the decrease of the Hα emission accompanied by the enhancement of the emission at the red wing. These short-term and periodic phenomena can be fairly explained with the X-ray induced H α emission by employing the orbital solution deduced from near-infrared light curves (Marti & Paredes 1995). Thus provides some evidence for a correlation with the orbital motion of the neutron star.