Orbital and Superorbital Monitoring of the Be/X-ray binary A0538-66: constraints on the system parameters

TL;DR

This study combines long-term photometry and high-resolution spectroscopy to refine the orbital parameters and system properties of the Be/X-ray binary A0538-66, revealing high eccentricity and constraints on the donor star's mass.

Contribution

It provides detailed constraints on the orbital parameters and system properties of A0538-66 using combined photometry and spectroscopy, refining previous measurements.

Findings

Orbital period refined to 16.6409 days.

High orbital eccentricity of 0.72 confirmed.

Donor star likely undermassive for its spectral type.

Abstract

We combine the decade long photometry of the Be/X-ray binary system A0538-66 provided by the MACHO and OGLE IV projects with high resolution SALT spectroscopy to provide detailed constraints on the orbital parameters and system properties. The ~420d superorbital modulation is present throughout, but has reduced in amplitude in recent years. The well-defined 16.6409d orbital outbursts, which were a strong function of superorbital phase in the MACHO data (not occurring at all at superorbital maximum), are present throughout the OGLE IV coverage. However, their amplitude reduces during superorbital maximum. We have refined the orbital period and ephemeris of the optical outburst based on ~25 yrs light curves to HJD = 2455674.48 +/- 0.03 + n*16.6409 +/- 0.0003d. Our SALT spectra reveal a B1 III star with vsini of 285 km/s from which we have derived an orbital radial velocity curve which confirms the high eccentricity of e = 0.72 +/- 0.14. Furthermore, the mass function indicates that, unless the neutron star far exceeds the canonical 1.44 Msun, the donor must be significantly undermassive for its spectral type. We discuss the implications of the geometry and our derived orbital solution on the observed behaviour of the system.