Correlated Timing and Spectral Behavior of 4U 1705−44

We follow the timing properties of the neutron star low-mass X-ray binarysystem 4U 1705-44 in different spectral states, as monitored by the Rossi X-rayTiming Explorer over about a month. We fit the power density spectra usingmultiple Lorentzians. We show that the characteristic frequencies of theseLorentzians, when properly identified, fit within the correlations previouslyreported. The time evolution of these frequencies and their relation with theparameters of the energy spectra reported in Barret & Olive (2002) are used toconstrain the accretion geometry changes. The spectral data were fitted by thesum of a blackbody and a Comptonized component and were interpreted in theframework of a truncated accretion disk geometry, with a varying truncationradius. If one assumes that the characteristic frequencies of the Lorentziansare some measure of this truncation radius, as in most theoretical models, thenthe timing data presented here strengthen the above interpretation. The soft tohard and hard to soft transitions are clearly associated with the disk recedingfrom and approaching the neutron star respectively. During the transitions,correlations are found between the Lorentzian frequencies and the flux andtemperature of the blackbody, which is thus likely to be coming from the disk.On the other hand, in the hard state, the characteristic Lorentziansfrequencies which are at the lowest, remained nearly constant despitesignificant evolution of the spectra parameters. The disk no longer contributesto the X-ray emission, and the blackbody is now likely to be emitted by theneutron star surface which is providing the seed photons for theComptonization.Comment: 20 pages, 8 figures, 2 tables, to appear in the Astrophysical Journal (see also astro-ph/0205184