An Atlas of Burst Oscillations and Spectral Properties in 4U 1728−34

We study correlations between the burst properties and the position in thecolor-color diagram of the low mass X-ray binary 4U 1728-34 using 21 burstsfound in two data sets obtained with the Rossi X-ray Timing Explorer. Usingspectral fits we analyze the spectral evolution during the bursts and determinethe burst peak flux, temperature, rise time and fluence. Using dynamical powerspectra we study the properties of the ~363 Hz burst oscillations. We findcorrelations between fluence, peak flux, the occurrence of radius expansion andthe presence of burst oscillations, and the position of the source in thecolor-color diagram. The radius expansion bursts with and without burstoscillations differ both with respect to where they occur in the color-colordiagram and in how the radius expansion takes place. We compare our resultswith those of a similar study by Muno et al. (2000) for KS 1731-26. Both KS1731-260 and 4U 1728-34 show more or less the same behavior with respect to thedependence of the presence/absence of the burst oscillations on the position ofthe source in the color-color diagram, but the dependence of most of thespectral burst parameters on the position in the color-color diagram is clearlydifferent. We find that the systematic absence in 4U 1728-34 of burstoscillations in the ``island'' state of the color-color diagram is notexplained by the bursts involving mixed H-He burning, as may be the case in KS1731--260 (Cumming & Bildsten 2000).Comment: 23 pages, 22 figures, ApJ accepte