SXP 15.6 -- an accreting pulsar close to spin equilibrium?

TL;DR

SXP 15.6 is a Be star X-ray binary in the SMC with a neutron star nearly at spin equilibrium, showing minimal spin change due to balanced angular momentum transfer and radiative losses, supported by multi-wavelength observations.

Contribution

This study provides the first detailed analysis showing SXP 15.6's neutron star is exceptionally close to spin equilibrium, unlike other systems in the SMC.

Findings

Neutron star in SXP 15.6 is near spin equilibrium.

X-ray outburst observed with multi-wavelength support.

Historical data confirms long-term stability in spin behavior.

Abstract

SXP 15.6 is a recently established Be star X-ray binary system (BeXRB) in the Small Magellanic Cloud (SMC). Like many such systems the variable X-ray emission is driven by the underlying behaviour of the mass donor Be star. It is shown here that the neutron star in this system is exceptionally close to spin equilibrium averaged over several years, with the angular momentum gain from mass transfer being almost exactly balanced by radiative losses. This makes SXP 15.6 exceptional compared to all other known members of its class in the SMC, all of whom exhibit much higher spin period changes. In this paper we report on X-ray observations of the brightest known outburst from this system. These observations are supported by contemporaneous optical and radio observations, as well as several years of historical data.