Low mass X-ray binaries are two gravitationally bound stars, where the secondary is a normal solar type like star and the primary a compact object, i.e. a black hole or neutron star. These systems are excellent laboratories to test several theories, includ ing stellar evolution, theory of accretion and the theory of relativity. However, accurate measurements of the masses of the two components in these binaries are necessary to test these theories, and that has been complicated since the secondary is many magnitudes fainter than the accretion disk around the compact object. We propose a new technique to measure the masses more accurately than has been possible thus far. Recent observations have shown that a signature of the secondary is present in certain fluorescence lines, in particular the Bowen fluorescence lines around 4640 angstroms. Furthermore, the observed optical emission is mainly due to reprocessing of the X-rays, and any variability observed in the X-rays should also be present in the optical , but delayed with a few seconds depending on the reprocessing time and distance of the optical site compared to that of the X-rays. Simultaneous X-ray and high temperal, high resolution optical spectroscopic observations, should be able to measure the de lay in the fluorescence lines, and thereby constrain the system parameters of low mass X-ray binaries. For these observations large, 10 m class, telescopes with spectrographs that have high timing capabilities are necessary, and those will only become avai lable by the end of 2005 when GranteCan and SALT come on-line.
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