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-massX-ray binary 4U1705-44 observed by the Rossi X-ray Timing Explorer. In theX-ray color-color diagram (CCD), the source, classified as an Atoll, samplesthe upper parts of a Z, starting from and returning to the left bottom of theZ. We follow the path of 4U1705-44 on its CCD, model its broad band X-ray/hardX-ray spectrum, and compute the Fourier power density spectrum of its X-rayvariability. The spectral transitions of 4U1705-44 are shown to be primarilyassociated with changes in the temperature of the Comptonizing electrons. Inthe hard state (top branch of the Z), the source evolves from left to right onthe CCD, while its luminosity smoothly increases. Our observations can be interpreted in the framework of a model made of atruncated accretion disk of varying inner radius and an inner flow mergingsmoothly with the neutron star boundary layer. The spectral evolution could bedriven by changes in the truncation radius of the disk; e.g. the soft to hardtransition could be caused by the disk moving outwards. If this model iscorrect, then our data show that the disk truncation radius is not set by theinstantaneous mass accretion rate, as derived from the source bolometricluminosity. Comparing the power density spectra of 4U1705-44 and Z sources when theyoccupy similar branches of the Z, we show that the most significant differenceis on the diagonal branch, on which the power density spectra of 4U1705-44remain similar to the ones measured on the top branch of the Z (hard state).Comment: 23 pages, 11 figures, 2 tables, to appear in the Astrophysical Journa