The N Enrichment and Supernova Ejection of the Runaway Microquasar LS 5039

Abstract

We present an investigation of new optical and ultraviolet spectra of the mass donor star in the massive X-ray binary LS 5039. The optical band spectral line strengths indicate that the atmosphere is N-rich and C-poor, and we classify the stellar spectrum as type ON6.5 V((f)). The N-strong and C-weak pattern is also found in the stellar wind P Cygni lines of N V λ1240 and C IV λ1550 (narrow absorption components in the former indicate that the wind terminal velocity is V∞ = 2440 ± 190 km s-1). We suggest that the N enrichment may result from internal mixing if the O star was born as a rapid rotator or the O star may have accreted N-rich gas prior to a common envelope interaction with the progenitor of the supernova. We reevaluated the orbital elements to find an orbital period of P = 4.4267 ± 0.0005 days. We compared the spectral line profiles with new non-LTE line-blanketed model spectra from Lanz and Hubeny, from which we derive an effective temperature Teff = 37.5 ± 1.7 kK, gravity log g = 4.0 ± 0.1, and projected rotational velocity V sin i = 140 ± 8 km s-1. We fitted the UV, optical, and IR flux distribution by using a model spectrum and extinction law with parameters E(B - V) = 1.28 ± 0.02 and R = 3.18 ± 0.07. We confirm the covariability of the observed X-ray flux and stellar wind mass-loss rate derived from the Hα profile, which supports the wind accretion scenario for the X-ray production in LS 5039. Wind accretion models indicate that the compact companion has mass MX/M☉ = 1.4 ± 0.4, consistent with its identification as a neutron star. We argue that the O star has mass in the range 20-35 M☉ (based on a lower limit for the distance and the lack of eclipses). The observed eccentricity and runaway velocity of the binary can be reconciled only if the neutron star received a modest kick velocity due to a slight asymmetry in the supernova explosion (during which more than 5 M☉ was ejected).