Hard X-ray light curves of high-mass X-ray binaries

Using the 9 years of continuous data now available from the Burst and Transient Source Experiment (BATSE) aboard CGRO, we have measured orbital periods and produced folded light curves for eight high-mass X-ray binaries (HMXB). Given the length of the data sets, our determinations are based on many more binary orbits than previous investigations. Thus our source detections have a high statistical significance and we are able to follow long-term trends in X-ray output. In particular, we focus on two systems: A0538-668 and EXO2030+375, both HMXBs exhibiting Type I outbursts. Recent work on A0538-668 reported a 16.65-d optical variability caused by the orbital period, but only seen during minima of a longer-term variability at 421 d. We searched for this signal in the BATSE data set but we found no evidence for such a modulation and place an upper limit of 3.0 × 10−3 photon cm−2 s−1 in the 20–70 keV BATSE energy band, based upon statistical modelling of the signal. Previous observations of EXO2030+375 using RXTE ASM data indicate secondary outbursts occur at apastron passage. We present a light curve for an earlier epoch, showing convincing evidence for such apastron outbursts. We find apastron outbursts in three sources, all having orbital periods greater than 41 d. No such signal is conclusively detected in the more rapidly orbiting systems studied.