Eclipse Observations of EXO0748-676 and X1658-298 with USA and RXTE

The orbital period derivative is the most critical diagnostic of LMXB evolution. We now have over 130 USA and RXTE mid-eclipse time measurements in hand for the LMXB EXO0748-676, and approximately 50 measured mid-eclipse times for the transient LMXB X1658-298. For EXO0748-676 we find a large positive orbital period derivative and a rapid time scale for orbital evolution (τ _orb = 2.0 x 10⁷ yr). 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 τ _orb = 10⁸ yr. The positive orbit period derivative 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 20 years. We will update the orbital ephemerides for both systems and report the results of our program of eclipse observations of X1658-298 and EXO0748-676 with USA and RXTE. This research is supported by the Office of Naval Research.