Orbital parameters of the microquasar LS I +61 303

Abstract

New optical spectroscopy of the high-mass X-ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5-4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes 65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i 113 km s -1 .