An analysis of optical periods in individual Small Magellanic Cloud Be/X-ray pulsars

A detailed analysis of optical photometry for 51 Small Magellanic Cloud Be/X-ray pulsars has been undertaken with the goal of better understanding their modes of variability. Because two pulsars are tentatively associated with the same optical counterpart, the photometric behaviour of 50 unique sources is discussed. We find that 22 systems show signatures consistent with orbital periods, and 29 systems exhibit non-radial pulsations (6 of these sources show both types of variation). No clear photometric period could be identified for five systems. There is a slight tendency for optical pulsations to be found in redder and/or fainter systems, while the orbital periods do not depend on system colour or magnitude. The period analysis of some sources is complicated by the presence of a changing pulsation period or by a period that is shorter than the daily sampling interval in the photometric surveys. We use synthetic data sets to illustrate the difficulty in solving these problems. Consequently, for a few systems, we find that previously determined `orbital' periods are most likely aliases of non-radial pulsation periods.