SIMBAD references

Using archival RXTE data we derive the 2-16 keV Fourier-resolved spectra (FRS) of the atoll source 4U 1728-34 in a sequence of its timing states as its low quasi-periodic oscillation (QPO) frequency spans the range between 6 and 94 Hz. The increase in the QPO frequency accompanies a spectral transition of the source from its island to its banana states. The banana-states' FRS are well fitted by a single blackbody component with kT∼2-3 keV depending on the source position in the color-color diagram and the Fourier frequency, thus indicating that this spectral component is responsible for the source variability on these timescales. This result is in approximate agreement with similar behavior exhibited by the Z sources, suggesting that, as in that case, the boundary layer–the likely source of the thermal component–is supported by radiation pressure. Furthermore, it is found that the iron line at ∼6.6 keV, clearly present in the averaged spectra, not apparent within the limitations of our measurements in the frequency-resolved spectra irrespective of the frequency range. This would indicate that this spectral component exhibits little variability on timescales of 10^–2-10² s. In the island state the single blackbody model proved inadequate, particularly notable in our lowest frequency band (0.008-0.8 Hz). An absorbed power law or an additive blackbody plus hard power-law model was required to obtain a satisfactory fit. Statistics do not allow unambiguous discrimination between these possible scenarios.