Preliminary determinations of the masses of the neutron star and mass donor in the High Mass X-ray Binary system EXO 1722-363

Original · EN

We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO1722-363 and hence determine the stellar masses of the components. We used a set of archival K s-band infrared spectra of the counterpart to EXO1722-363 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 ± 5.0 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 EXO1722-363. The X-ray luminosity for this distance range is 4.7 × 10³⁵ ≤ L X ≤ 9.2 × 10³⁶ erg s⁻¹. EXO1722-363 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 EXO1722-363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy.