Research Note

Preliminary determinations of the masses of the neutron star and mass donor in the high mass X-ray binary system EXO 1722–363^*

¹ Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK e-mail: a.mason@open.ac.uk
² Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain
³ School of Physics & Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, UK
⁴ Division of Earth, Space & Environment, University of Glamorgan, Pontypridd, CF37 1DL, UK

Received: 1 October 2009
Accepted: 17 November 2009

Abstract

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 -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  km s^-1. When combined with other measured parameters of the system, this yields masses in the range for the neutron star and for the B0–1 Ia supergiant companion. These lower and upper limits were obtained under the assumption that the system is viewed edge-on () for the lower limit and the supergiant fills its Roche lobe () for the upper limit respectively. The system inclination is constrained to and the Roche lobe-filling factor of the supergiant is . Additionally we were able to further constrain our distance determination to be  kpc for EXO 1722–363. The X-ray luminosity for this distance range is  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.