Very Long-term Optical Variability of High Mass X-ray Binaries in the SMC

TL;DR

This study analyzes over 16 years of optical data from Be X-ray binaries in the SMC, revealing superorbital variations, orbital periodicities, and their correlations, providing insights into circumstellar disc dynamics.

Contribution

It presents the first long-term optical variability analysis of SMC Be X-ray binaries, identifying superorbital and orbital periodicities and their interrelations.

Findings

Most sources show superorbital variations related to disc formation and depletion.

Detected many optical orbital periodicities as regular outbursts.

Found a correlation between outburst amplitude and brightness, possibly due to orbital inclination.

Abstract

We have studied the very long-term temporal properties of the optical emission from Be X-ray binaries (BeX) in the Small Magellanic Cloud over a ~ 16 yr baseline, using light curves from the MACHO and OGLE databases. All the BeX in our sample display superorbital variations, many of them quasi-periodic on timescales of ~ 200-3000 d. These long-term variations are believed to be related to the formation and depletion of the circumstellar disc around the Be star and we compare and contrast their behaviour with that of the LMC's prototypical BeX, A0538-66. The great majority of sources show a correlation of outburst amplitude with brightness (the opposite to that seen in A0538-66) although the amplitudes are mostly small (< 0.1 mag). We suggest this is an orbital inclination effect. In addition, we have also detected many of their optical orbital periodicities, visible as a series of precisely regular outbursts. Furthermore, the amplitude of these periodic outbursts can vary through the long-term superorbital cycle, and we discuss mechanisms which can produce this effect, as well as examining an apparent correlation between these periodicities. As a by-product of this variation survey we have compiled a list of all the reported SMC BeX orbital and superorbital periodicities at optical and X-ray wavelengths.