Bowen Fluorescence from the Companion Star in X1822-371

Authors:
J. Casares,
D. Steeghs,
R.I. Hynes,
P.A. Charles,
K. O’Brien

Comments: 19 pages, 6 figures. accepted by ApJ

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 lambda 4640 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 K2 > 300 +_ 8 km/s, which, combined with a previous determination of the
inclination angle and the pulsar’s radial velocity curve, yield M2 > 0.36(2)
Msun and M1 > 1.14(6) Msun. The HeI lambda 4471 absorption line moves at lower
velocities (~ 225 km/s) and with a -0.05 phase shift, suggests a likely origin
on the gas stream near the L1 point. In addition, we detect an S-wave emission
of OVI lambda 3811 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
gamma = -44 +_ 5 km/s based on the motion of the HeII lambda 4686 wings and
Doppler tomography. This work confirms the power of imaging the companion stars
in LMXBs and outbursting transients using the Bowen fluorescence transitions.

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