The iron K-shell features of MXB 1728-34 from a simultaneous Chandra-RXTE observation

We report on a simultaneous Chandra and RossiXTE observation of the low-mass X- ray binary atoll bursting source MXB 1728 - 34 performed on 2002 March 3 - 5. We fit the 1.2 - 35 keV continuum spectrum with a blackbody plus a Comptonized component. Large residuals at 6 - 10 keV can be fitted by a broad (FWHM similar or equal to 2 keV) Gaussian emission line or, alternatively, by two absorption edges associated with lowly ionized iron and Fe XXV/ XXVI at similar to 7.1 keV and similar to 9 keV, respectively. In this interpretation, we find no evidence of broad emission lines between 6 and 7 keV. We test our alternative model of the iron K shell region by reanalysing a previous BeppoSAX observation of MXB 1728 - 34, finding a general agreement with our new spectral model.