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Session 36 - New Insights on X-ray Binaries - I.
Oral session, Wednesday, June 11
North Main Hall C/D,
We report on an analysis of the spectra from X-ray bursts observed with RXTE from the low mass X-ray binary 4U 1728-34. Of 12 bursts observed 8 showed radius expansion, that is, their peak fluxes reached the Eddington limit. In general, the radius expansion bursts with the largest fluences reached higher peak fluxes, ie., they had higher local Eddington fluxes. These brightest bursts ocurred when the persistent X-ray flux was near the lowest observed. We fit the observed flux versus color temperature during burst decay to theoretical values and find variations which can be attributed to an increase in the hydrogen abundance as the burst cools. This could be due to initial loss via radiation pressure and subsequent accretion later in the burst of the hydrogen-rich shell. Such mass loss had been proposed earlier by Ebisuzaki amp; Nakamura (1988). We discuss the mass-radius relations derived assuming that the atmospheres are essentially hydrogen poor during the early post-peak cooling portions of the brightest bursts.
