ROSAT measurement of the evolving orbital period in the LMXB EXO0748-676

Abstract

The sign and magnitude of the orbital period derivative are perhaps the most critical diagnostics of the evolution of low mass x‐ray binaries (LMXBs). However ≲4 LMBXs have measured orbital period derivatives. One of these is the eclipsing x‐ray binary EXO0748‐676, whose sharp edged eclipse transitions provide a good fiducial marker. We have observed a single eclipse egress with ROSAT. We have combined our data with EXOSAT and Ginga timings to fit the ephemeris of EXO0748‐676. Assuming that the observed eclipse times are determined solely by a deterministic period plus measurement error, we confirm Asai et al.—a constant period and a constant period derivative are both ruled out. A constant second period derivative and a 12 yr sinusoidal variability provide equally good fits. The sinusoidal variation in orbital period is reminiscent of several cataclysmic variables. The period could be interpreted as evidence of a third body or of spin‐orbit coupling. However no CV has been observed for more than 1.5 long cycles, and many show orbital period timings that deviate significantly from the predictions of the sinusoidal ephemeris. We wonder if there might not be intrinsic, stochastic jitter in the time of individual eclipses. If so, the O‐C diagram represents a random walk in orbital phase, and it will usually be well fitted by a sinusoid with a period 1–2 times the duration of the observational history. We present statistical tests in support of this interpretation.