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M. T. Wolff, P. S. Ray, K. S. Wood (Naval Research Laboratory), A. P. Smale (GSFC/USRA)
The orbital period derivative is a critical diagnostic of LMXB evolution. We now have 182 RXTE mid-eclipse time measurements in hand for the LMXB EXO0748-676. When combined with earlier measurements from other satellites we have a total of over 230 mid-eclipse timings for this source. For the transient LMXB X1658-298 we have 50 measured mid-eclipse times. For EXO0748-676 we find that a piecewise linear constant period model for the orbit period evolution between the discovery in 1985 and the present shows remarkable behavior. The EXO0748-676 orbital period appears to increase near MJD 48000 by ~8 msec but then appears to decrease again near MJD 51700 by ~6.7 msec! In the case of X1658-298 our observations show that the 7.1-hr orbital period of this system is decreasing with a time scale of \tauorb ~4-10 x 107 yr. The orbit period behavior in EXO0748-676 is difficult to understand in the context of current models of LMXB evolution. In the case of X1658-298 the period derivative appears close to values predicted by some LMXB evolutionary models. However, the magnitude of the orbital period derivative for this source is still difficult to understand because little or no mass transfer is thought to have occurred while the system was in quiescence during the interval 1978-1999. We apply maximum likelihood statistical methods to these data to quantify the intrinsic phase variability and the orbital period variability for both systems over the past 22 years. We report the latest results of our program of eclipse observations of EXO0748-676 and X1658-298 with RXTE.
This research is supported by the Office of Naval Research and the NASA Astrophysical Data Program.
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Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.