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A.B. Giles (UTas), K.M. Hill (UTas), T.E. Strohmayer (LHEA/GSFC USRA)
The 581 Hz oscillations seen during Type I X-ray bursts from 4U1636-53 sometimes appear to drift in frequency towards an asymptotic limit. The limiting frequencies vary slightly and a possible explanation is that this is due to an orbital doppler shift of order 5.8 x 10-4. The orbital phases of the X-ray bursts can in principle be estimated from the binary optical light curve modulation.
New optical light curves obtained in March/April 1998 have been used to update the previous orbital ephemeris of van Paradijs et al. (A&A, 1980) and to provide estimates for the binary phases of three suitable X-ray bursts. Our light curves were not compatible with the van Paradijs ephemeris or with a substantial correction to it (1 part in 355) by Augusteijn et al. (A&A, 1998). Our derived ephemeris is consistent with the doppler model interpretation but the Augusteijn ephemeris is not. The difference in the two possible periods is so large that further observations were made in March 1999 to differentiate between the two possibilities when the 'sine curve' predictions were in antiphase. The outcome is presented in this paper. More RXTE observations of 4U1636-53 are being obtained during 1999 to get a larger sample of X-ray bursts. Coordinated photometry and spectroscopy will also take place in mid-1999. If a doppler interpretation is confirmed this offers the prospect of determining the X-ray mass function for this and similar systems once enough suitable X-ray bursts have been detected.
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