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

Abstract

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${}_{\mathrm{s}}$-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 $24.5\pm 5.0$ km s${}^{-1}$. When combined with other measured parameters of the system, this yields masses in the range 1.5 $\pm$ 0.4 - 1.6 $\pm$ 0.4 M${}_{\odot }$ for the neutron star and 13.6 $\pm$ 1.6 - 15.2 $\pm$ 1.9 M${}_{\odot }$ 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${}^{\circ }$) for the lower limit and the supergiant fills its Roche lobe ($\beta =1$) for the upper limit respectively. The system inclination is constrained to $i>75^{\circ }$ and the Roche lobe-filling factor of the supergiant is $\beta >0.9$. Additionally we were able to further constrain our distance determination to be 7.1 $\le$ d $\le$ 7.9 kpc for EXO1722-363. The X-ray luminosity for this distance range is 4.7 $\times$ 10${}^{35}$ $\le$ L${}_{\mathrm{X}}$ $\le$ 9.2 $\times$ 10${}^{36}$ 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.

4 pages, 3 figures. Accepted for Publication in Astronomy and Astrophysics

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