Eclipse Timings of the Low‐Mass X‐Ray Binary EXO 0748−676. III. Orbital Period Jitter Observed with the Unconventional Stellar Aspect Experiment and theRossi X‐Ray Timing Explorer

We present seven eclipse timings of the low-mass X-ray binary (LMXB) EXO 0748-676 obtained with the Unconventional Stellar Aspect (USA) Experiment during 1999-2000, as well as 122 eclipse timings obtained with the Rossi X-Ray Timing Explorer ( RXTE ) during 1996-2000. According to our analysis, the mean orbital period has increased by ~8 ms between the pre- RXTE era (1985-1990) and the RXTE /USA era (1996-2000). This corresponds to an orbital period derivative of P orb / orb ~ 2 × 10 7 yr. However, neither a constant orbital period derivative nor any other simple ephemeris provides an acceptable fit to the data; individual timings of eclipse centers have residuals of up to 15 or more seconds away from our derived smooth ephemerides. When we consider all published eclipse timing data, including those presented here, a model that includes observational measurement error, cumulative period jitter, and underlying period evolution is found to be consistent with the timing data. We discuss several physical mechanisms for LMXB orbital evolution in an effort to account for the change in orbital period and the observed intrinsic jitter in the mideclipse times.