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 color-color diagram , model its broad band X-ray/hard X-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 \sim 10 to \sim 20 % .
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 \sim 2.1 to \sim 0.7 \times 10 ^ { 37 } ergs s ^ { -1 } , whereas the hard to soft transition took place at higher luminosities between \sim 2.5 and \sim 3.1 \times 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 of the blackbody temperature , while the electron temperature remained remarkably constant ( \sim 13 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 outwards .
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 ) .