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Session 44 - Stellar X-Ray Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,
EXO 0748-676 is the only X-ray active, eclipsing, low mass X-ray binary. Eclipses have been timed for more than a decade. We observed 10 eclipses in May 1996 and August 1996 with the Rossi X-ray Timing Explorer and obtained mid-eclipse timings with <0.5 s accuracy. This brings the number of mid-eclipse timings for EXO 0748-676 to 54; they span 26,358 cycles of the 3.82 hr binary period.
The RXTE eclipses are observed 60-70 s later than expected by extrapolating ephemerides based on pre-RXTE observations. The triple system model proposed by Asai et al (1992) and Corbet et al (1995) to explain eclipse timing residuals is rejected. The orbital period measured during mid-1996 with RXTE data is 11\pm1 ms longer than the mean period during 1985-1991. If a constant orbital period derivative (quadratic ephemeris) is fit to the data, then P_orb / \dot P_orb = 1.2\times 10^7 yr. The apparent change in period can be explained ad hoc by either a series of mass ejections from the main sequence secondary or by the transfer of rotational angular momentum from the neutron star primary to the orbit. However any smooth ephemerides (including these two models) have significant timing residuals. These additional timing residuals can be explained by stochastic variability in eclipse timings due to changes in the companion star's atmosphere which serves as an occulting edge (Hertz et al 1995) or by episodic mass ejections. For the first time we have directly observed this variability in consecutive eclipses of EXO 0748-676.
