Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic

We have computed, based on 17 infrared radial velocities, the first set of orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The giant’s companion is a neutron star, the bright X-ray source GX 1+4. We rule out the previously proposed period of 304 days, and instead, find an orbital period of 1161 days, by far the longest of any known X-ray binary. The orbit has a modest eccentricity of 0.10 with an orbital circularization time of � 5 × 10 6 years. The large mass function of the orbit significantly restricts the mass of the M giant. Adopting a neutron-star mass of 1.35 M⊙, the maximum mass of the M giant is 1.22 M⊙, making it the less massive star. Spectrum synthesis analysis of several infrared spectral regions results in slightly subsolar abundances for most metals. Carbon and nitrogen are in the expected ratio resulting from the redgiant first dredge-up phase. The lack of 17 O suggests that the M-giant has a mass less than 1.3 M⊙, consistent with our maximum mass. The surface gravity and maximum mass of the M giant result in a radius of 103 R⊙, much smaller than its