M ar 2 00 3 Bowen Fluorescence from the Companion Star in X 1822 - 371

– 2 – We present a clear evidence for the motion of the companion star in the Low Mass X-Ray Binary (LMXB) X1822-371. We detect NIII λ4640 emission moving in antiphase with the radial velocity curve of the neutron star and produced on the X-ray heated hemisphere of the donor star. From the motion of this feature we derive a lower limit to the radial velocity semi-amplitude of the companion star K 2 ≥ 300 ± 8 km s −1 , which, combined with a previous determination of the inclination angle and the pulsar's radial velocity curve, yield M 2 ≥ 0.36(2)M ⊙ and M 1 ≥ 1.14(6)M ⊙. The HeI λ4471 absorption line moves at lower velocities (≃ 225 km s −1) and with a-0.05 phase shift, suggests a likely origin on the gas stream near the L 1 point. In addition, we detect an S-wave emission of OVI λ3811 produced by illumination of the hot-spot bulge by the central source. The Balmer lines are dominated by broad absorptions probably due to obscuration of the accretion disc by vertically extended cool material from the splash region and overflowing stream. We also derive a more accurate, and significantly different (compared to earlier work) systemic velocity of γ = −44 ± 5 km s −1 based on the motion of the HeII λ4686 wings and Doppler tomography. This work confirms the power of imaging the companion stars in LMXBs and outbursting transients using the Bowen fluorescence transitions.