Mason, A. B.; Norton, A. J.; Clark, J. S.; Negueruela, I. and Roche, P.
Preliminary determinations of the masses of the neutron star and mass donor in the high mass X-ray binary system EXO 1722-363.
Astronomy & Astrophysics, 509, article no. A79.
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Aims. 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.
Methods. We used a set of archival Ks-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.
Results. The resulting radial velocity curve has a semi-amplitude of 24.5 ± 5.0 km s-1. When combined with other measured parameters of the system, this yields masses in the range 1.5 ± 0.4-1.6 ± 0.4 M⊙ for the neutron star and 13.6 ± 1.6-15.2 ± 1.9 M⊙ 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 ≤ 7.9 kpc for EXO 1722–363. The X-ray luminosity for this distance range is 4.7 x 1035 ≤ 9.2 x 1036 erg s-1.
Conclusions. 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.
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