ar X iv : a st ro - p h / 04 11 27 0 v 2 2 2 D ec 2 00 4 High Resolution Spectroscopy of 4 U 1728 - 34 from a Simultaneous Chandra - RXTE Observation

We report on a simultaneous Chandra and RossiXTE observation of the LMXB atoll bursting source 4U 1728-34 performed on 2002 March 3–5. We fitted the 1.2–35 keV continuum spectrum with a blackbody plus a Comptonized component. An overabundance of Si by a factor of ∼ 2 with respect to Solar abundance is required for a satisfactory fit. Large residuals at 6–10 keV can be fitted by a broad (FWHM ≃ 1.6 keV) Gaussian emission line, or, alternatively, by absorption edges associated with Fe I and Fe XXV at ∼ 7.1 keV and∼ 9 keV, respectively. In this interpretation, we find no evidence of broad, or narrow Fe Kα line, between 6 and 7 keV. We tested our alternative modeling of the iron Kα region by reanalyzing a previous BeppoSAX observation of 4U 1728-34, finding a general agreement with our new spectral model.