A peculiar spectral state transition of 4U 1705-44: When an atoll looks like a Z

We report on a clear spectral state transition of the neutron star low-mass X-ray binary 4U 1705-44 observed by the Rossi X-Ray Timing Explorer. In the X-ray color-color diagram (CCD), the source, classified as an atoll, samples the upper parts of a Z, starting from and returning to the left bottom of the Z. We follow the path of 4U 1705-44 on its CCD, model its broadband X-ray/hardX-ray spectrum, and compute the Fourier power density spectrum of its X-ray variability. The energy spectrum can be described as the sum of a dominating Comptonized component, a blackbody, and a 6.4 keV iron line. During most observations, 4U 1705-44 displays strong band-limited noise, with an integrated fractional rms varying from similar to10% to similar to20%. The spectral transitions of 4U 1705-44 are shown to be primarily associated with changes in the temperature of the Comptonizing electrons. During the soft-to-hard transition, the source luminosity decreased from similar to2.1 to similar to0.7 x 10(37) ergs s(-1), whereas the hard-to-soft transition took place at higher luminosities between similar to2.5 and similar to3.1 x 10(37) ergs s(-1). In the hard state ( top branch of the Z), the source evolves from left to right on the CCD, while its luminosity smoothly increases. Along this branch, the increase in the soft color is related to a smooth increase in the blackbody temperature, while the electron temperature remained remarkably constant (similar to13 keV). Our observations can be interpreted in the framework of a model made of a truncated accretion disk of varying inner radius and an inner flow merging smoothly with the neutron star boundary layer. The spectral evolution could be driven by changes in the truncation radius of the disk; e. g., the soft-to-hard transition could be caused by the disk moving outward. If this model is correct, then our data show that the disk truncation radius is not set by the instantaneous mass accretion rate, as derived from the source bolometric luminosity. Comparing the power density spectra of 4U 1705-44 and Z sources when they occupy similar branches of the Z, we show that the most significant difference is on the diagonal branch, on which the power density spectra of 4U 1705-44 remain similar to the ones measured on the top branch of the Z (hard state).