Short-term pulse frequency fluctuations of OAO 1657-415 from RXTE observations

We present new X-ray observations of the high-mass X-ray binary (HMXRB) pulsar OAO 1657-415, obtained during one orbital period (10.44 d) with the Rossi X-Ray Timing Explorer (RXTE). Using the binary orbital parameters, obtained from Burst and Transient Source Experiment (BATSE) observations, we resolve the fluctuations in the pulse frequency at time-scales on the order of 1 d for the first time. Recent BATSE results by Baykal showed that OAO 1657-415 has spin-up/down trends in its pulse frequency time series, without any correlation with the X-ray luminosity at energies > 20 keV. In the present RXTE observations the source is found to be in an extended phase of spin-down. We also find a gradual increase in the X-ray luminosity which is correlated with a marginal spin-up episode. The marginal correlation between the gradual spin-up (or decrease in spin-down rate) and increase in X-ray luminosity suggests that OAO 1657-415 is observed during a stable accretion episode where the prograde accretion disc is formed.