The Symbiotic Neutron Star Binary GX 1+4/V2116 Ophiuchi

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Abstract
We present extensive optical, infrared, and X-ray observations of the S-type symbiotic low-mass X-ray binary GX 1+4/V2116 Oph, which consists of a 2 minute X-ray pulsar accreting from an M6 III giant. This is the only symbiotic system definitely known to contain a neutron star. The mean observed spectral type of the X-ray-heated mass donor is M5 III. The steady interstellar extinction toward the binary (AV = 5.0 ± 0.6) contrasts the variable hydrogen column density inferred from X-ray measurements, most likely evidence for a variable stellar wind. The mass donor is probably near the tip of the first-ascent red giant branch, in which case the system is 3-6 kpc distant and has an X-ray luminosity of ~1037 ergs s-1. It is also possible, though less likely, that the donor star is just beginning its ascent of the asymptotic giant branch, in which case the system is 12-15 kpc distant and has an X-ray luminosity of ~1038 ergs s-1. However, our measured AV argues against such a large distance. We show that the dense (Ne ~ 109 cm-3) emission-line nebula enshrouding the binary is powered by ultraviolet radiation from an accretion disk. The emission-line spectrum constrains the temperature profile and inner radius of the disk (and thus the pulsar's magnetic field strength), and we mention the implications this has for explaining the accretion torque reversals observed in the pulsar. We also show that the binary period must be 100 days and is most likely 260 days, which makes GX 1+4 the only known low-mass X-ray binary with Porb > 10 days. If the mass donor fills its Roche lobe, the mass transfer rate must be highly super-Eddington, which requires considerable mass loss from the binary. We discuss the alternative possibility that the accretion disk forms from the slow, dense stellar wind expected from the red giant.
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