HEAD 2000, November 2000
Session 24. X-Ray Binaries - Timing and Transients
Display, Wednesday, November 8, 2000, 8:00am-6:00pm, Bora Bora Ballroom

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[24.04] An Apparent Orbital Period Glitch in the Low Mass X-ray Binary EXO0748-676 Observed with USA and RXTE

P. Hertz (NASA Headquarters), M. T. Wolff, R. M. Bandyopadhyay (NRL/NRC Cooperative Research Associate), D. Livingston (Pennsylvania State Univ.), P. S. Ray, K. S. Wood (NRL)

We present ~25 new eclipse timings of the low mass X-ray binary EXO0748-676 obtained with the USA experiment during 1999-2000 as well as ~115 eclipse timings obtained with RXTE during 1996-2000. The mean orbital period has increased by ~8 ms between the pre-RXTE era (1985-1990) and the RXTE/USA era (1996-2000). This corresponds to an orbital period derivative of ~2 x 10-11. However neither a constant orbital period derivative nor any other simple ephemeris provides an acceptable fit: individual eclipse timings have residuals of up to 10 or more seconds. Eclipse profiles vary significantly as well.

We conclude that mid-eclipse timings are not stable orbital phase markers.

Several toy models to account for the change in orbital period, including tidal coupling, mass expulsion (~7 x 10-8 solar masses over 15 years), spin down of either star (fractional change in frequency of ~10-5 for companion star or ~10-3 for neutron star over 15 years), and an asteroid crashing into the companion star (~2500 km radius falling from infinity), are presented. None of the models is satisfactory.

Basic research in X-ray Astronomy at the Naval Research is supported by the Office of Naval Research. This work has been supported by the National Aeronautics and Space Administration.

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