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 days) 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 one day for the first time. Recent BATSE results showed that OAO 1657-415 has spin-up/down trends in its pulse frequency time series, without any correlation with the X-luminosity at energies $>$20 keV (Baykal 1997). 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 the OAO 1657-415 is observed during a stable accretion episode where the prograde accretion disk is formed. 
Altan Baykal (Middle East Technical University, Physics Department)