Astrophysical parameters of LS 437 and the nature of X0726-260

TL;DR

This study characterizes the optical and X-ray properties of the atypical Be/X-ray binary X0726-260, revealing its unique early spectral type, stable circumstellar disk, and orbital modulation of X-ray emission over decades.

Contribution

It provides the first detailed optical spectroscopic analysis of LS 437, the optical counterpart, and constrains the orbital and stellar parameters of X0726-260, highlighting its unusual features.

Findings

LS 437 is an O7.5 Ve star with Teff = 36,000 K.

X0726-260 has a 34.5-day orbital period with orbitally modulated X-ray emission.

The circumstellar disk has remained stable for over 40 years.

Abstract

Be/X-ray binaries, the most common class of high-mass X-ray binaries, are characterised by OBe companions, but display a rich variety of X-ray behaviours. One of the most atypical systems is X0726-260, which also has the earliest optical counterpart among the whole Milky Way and Magellanic Cloud sample. We intend to improve the characterisation of the optical counterpart, LS 437, and to constrain the physical mechanisms responsible for the anomalous properties of X0726-260. We analyse high-quality, high-resolution optical spectroscopy of LS 437 with standard quantitative methodology to derive stellar parameters. We also make use of archival X-ray monitoring. We derive a moderate projected rotational velocity v sin i $\approx$ 155 km/s and a spectral type O7.5 Ve (Teff = 36 000 K), which makes LS 437 substantially earlier than any other Oe star in an X-ray binary. At this spectral type, the stellar wind likely contributes significantly to mass accretion, and the X-ray light curve is strongly suggestive of an orbitally modulated wind accretor. The source shows marked carbon depletion, whereas nitrogen is only slightly above solar abundance. LS 437 is the earliest Oe star known in the Galaxy, alongside HD 155806. Long-term X-ray lightcurves of X0726-260 strengthen the identification of a persistent 34.5 d periodicity as the orbital period, demonstrating that the X-ray emission is orbitally modulated and no X-ray outbursts have occurred over the past 30 years. Likewise, emission features in the optical spectrum indicate a remarkably stable circumstellar disk, with no sign of major structural changes over the past 40 years. All these characteristics set X0726-260 clearly apart from typical Be/X-ray binaries.