2010A&A...509A..79M

We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO 1722-363 and hence determine the stellar masses of the components. We used a set of archival K_s-band infrared spectra of the counterpart to EXO 1722-363 obtained using ISAAC on the VLT, and cross-correlated them in order to measure the radial velocity of the star. The resulting radial velocity curve has a semi-amplitude of 24.5 ±5.0km/s. When combined with other measured parameters of the system, this yields masses in the range 1.5±0.4-1.6±0.4M_☉ for the neutron star and 13.6±1.6-15.2±1.9M_☉ for the B0-1 Ia supergiant companion. These lower and upper limits were obtained under the assumption that the system is viewed edge-on (i=90°) for the lower limit and the supergiant fills its Roche lobe (β=1) for the upper limit respectively. The system inclination is constrained to i>75° and the Roche lobe-filling factor of the supergiant is β>0.9. Additionally we were able to further constrain our distance determination to be 7.1≤d≤7.9kpc for EXO 1722-363. The X-ray luminosity for this distance range is 4.7x10³⁵≤L_X≤9.2x10³⁶erg/s. EXO 1722-363 therefore becomes the seventh of the ten known eclipsing X-ray binary pulsars for which a dynamical neutron star mass solution has been determined. Additionally EXO 1722-363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy.