Very long-term optical variability of high-mass X-ray binaries in the Small Magellanic Cloud

We have studied the very long-term temporal properties of the optical emission from Be X-ray binaries (BeX) in the Small Magellanic Cloud (SMC) over a similar to 16 yr baseline, using light curves from the MAssive Compact Halo Objects (MACHO) and Optical Gravitational Lensing Experiment (OGLE) data bases. All the BeX in our sample display superorbital variations, many of them quasi-periodic on time scales of similar to 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 Large Magellanic Cloud (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.