Detection of a broad iron emission line and sub - millisecond quasiperiodic oscillations from the type I X - ray burster 4 U 1728 - 34 in a high state

We report results from simultaneous RossiXTE and BeppoSAX observations of the neutron-star x-ray binary and type I X–ray burster 4U 1728-34. The source was found in a high luminosity state, L X ∼ 0.1L Edd , and quasiperiodic oscillations were detected at 1284 ± 6 Hz, the highest frequency ever observed from this source. The x-ray spectrum shows a broad, FWHM ∼ 0.8 keV, iron K α fluorescence line. We discuss interpretations of the broad line and the quasiperiodic oscillations and how future simultaneous spectral and timing observations can be used to test these interpretations and, potentially, to estimate the mass of the compact object.