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60 Chapter 2. Multiwavelength approach to LS 5039
2.5.4 Radial velocity curve
Optical spectroscopy aimed to reveal the radial velocity curve of LS 5039 was carried
out by McSwain et al. (2001). They performed the observations during three runs
in 1998 August, 1999 June and 2000 October using the 0.9 m coudé feed telescope at
Kitt Peak National Observatory, that provided a resolution R = λ/δλ = 9500. The
obtained radial velocity data appeared to have minima every ∼ 4 days. Therefore,
the data set was then analyzed in the search for an orbital period, and the following
orbital elements were derived after a least-squares fit: P = 4.117 ± 0.011 d, T =
JD 2451822.12±0.09, K = 14.7±0.9 km s −1 , V0 = 4.6±0.5 km s −1 , e = 0.41±0.05,
w = 217 ± 9 ◦ , rms = 3.3 km s −1 , f(m) = 0.00103 ± 0.00020 M⊙, and a1 sin i =
1.09 ± 0.07 km s −1 . They also obtained a projected rotational velocity of V sin i =
131 ± 6 km s −1 . The epoch T corresponds to the time of periastron, and the epoch
of inferior conjunction of the optical star (which would correspond to the time of an
X-ray eclipse if one occurs) is 0.25 days later, corresponding to a phase of 0.06. We
show in Fig. 2.11 the radial velocity curve obtained by McSwain et al (2001).
RADIAL VELOCITY (km s -1
)
30
20
10
0
-10
-20
-30
0.0 0.5 1.0
ORBITAL PHASE
Figure 2.11: Radial velocity measurements and orbital solution plotted against orbital
phase. Phase zero corresponds to periastron (from McSwain et al. 2001).