BEPPOSAX and RXTE spectral study of the low-mass X-ray binary 4u 1705-44. Spectral hardening during the banana branch

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We analyze the X-ray spectra of the atoll 4U 1705-44 when the source undergoes the island-banana state transition. We use the Rossi X-ray Timing Explorer (RXTE) and BeppoSAX observations for this analysis. We demonstrate that the broad-band energy spectral distributions for all evolutinary states can be fitted by a model, consisting two Comptonized components. One arises from the seed photons coming from a neutron star (NS) atmosphere at a temperature kTₛ₁<1.5 keV (herein Comptb) and a second resulting from the seed photons of Tₛ₂ 1.1-1.3 keV coming from the disk (herein Comptb2). We found that we needed to add a low-temperature blackbody and an iron-line (Gaussian) component to the model in order to obtain high-quality fits. The data analysis using this model indicates that the power-law photon index Gamma₁ of our model is always about 2, independently of the spectral state. Another parameter, Gamma₂ demonstrates a two-phase behavior depending on the spectral state. Gamma₂ is quasi-constant at Gamma₂ 2 when the electron temperature kT⁽²⁾ₑ<80 keV and Gamma₂ is less than 2, in the range of 1.3<Gamma₂<2, when kT⁽²⁾ₑ>80 keV. This phase is similar to that was previously found in the Z-source Sco X-1. We interpret the decreasing index phase using a model in which a super-Eddington radiation pressure from the neutron star causes an expansion of the Compton cloud similar to that found previously in Sco X-1 during the Flaring branch.