2012MNRAS.422..199M

We report near-infrared radial velocity (RV) measurements of the recently identified donor star in the high mass X-ray binary (HMXB) system OAO 1657-415 obtained in the H band using ISAAC on the Very Large Telescope.

Cross-correlation methods were employed to construct a RV curve with a semi-amplitude of 22.1 ±3.5 km/s. Combined with other measured parameters of this system it provides a dynamically determined neutron star (NS) mass of 1.42±0.26 M_☉ and a mass of 14.3±0.8 M_☉for the Ofpe/WN9 highly evolved donor star.

OAO 1657-415 is an eclipsing HMXB pulsar with the largest eccentricity and orbital period of any within its class. Of the 10 known eclipsing X-ray binary pulsars OAO 1657-415 becomes the ninth with a dynamically determined NS mass solution and only the second in an eccentric system. Furthermore, the donor star in OAO 1657-415 is much more highly evolved than the majority of the supergiant donors in other HMXBs, joining a small but growing list of HMXBs donors with extensive hydrogen depleted atmospheres.

Considering the evolutionary development of OAO 1657-415, we have estimated the binding energy of the envelope of the mass donor and find that there is insufficient energy for the removal of the donor's envelope via spiral-in, ruling out a common envelope evolutionary scenario. With its non-zero eccentricity and relatively large orbital period the identification of a definitive evolutionary pathway for OAO 1657-415 remains problematic, we conclude by proposing two scenarios which may account for OAO 1657-415 current orbital configuration.