Timing Studies on RXTE Observations of SAX J2103.5+4545
  SAX J2103.5+4545 has been continuously monitored for $\sim $ 900 days by Rossi X-ray Timing Explorer (RXTE) since its outburst in July 2002. Using these observations and previous archival RXTE observations of SAX J2103.5+4545, we refined the binary orbital parameters and find the new orbital period as P= (12.66536 $\pm $ 0.00088) days and the eccentricity as 0.4055$\pm$ 0.0032. With these new orbital parameters, we constructed the pulse frequency and pulse frequency derivative histories of the pulsar and confirmed the correlation between X-ray flux and pulse frequency derivative presented by Baykal, Stark and Swank (2002). We constructed the power spectra for the fluctuations of pulse frequency derivatives and found that the power law index of the noise spectra is 2.13 $\pm$ 0.6. The power law index is consistent with random walk in pulse frequency derivative and is the steepest among the HMXRBs.   X-ray spectra analysis confirmed the inverse correlation trend between power-law index and X-ray flux found by Baykal, Stark and Swank (2002). 
A. Baykal (1), S. C. Inam (2), M. J. Stark (3), C. M. Heffner (3,4),   A. E. Erkoca (5), J. H. Swank (6) ((1) Physics Depart...