2015ApJ...808..142S

We analyze the X-ray spectra of the atoll 4U 1705-44 when the source undergoes the island-banana state transition. We use the RXTE and BeppoSAX observations for this analysis. We demonstrate that the broadband energy spectral distributions for all evolutinary states can be fitted by a model consisting of two Comptonized components. One arises from the seed photons coming from a neutron star (NS) atmosphere at a temperature kT_s1 ≲ 1.5 keV (herein Comptb1), and a second results from the seed photons of T_S2 ∼ 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 Γ₁ of our model is always about 2, independently of the spectral state. Another parameter, Γ₂, demonstrates a two-phase behavior depending on the spectral state. Γ₂ is quasi-constant at Γ₂ ∼ 2 when the electron temperature KT_e⁽²⁾ < 80 keV, and Γ₂ is less than 2, in the range of 1.3 < Γ₂ < 2, when KT_e⁽²⁾ > 80 keV. This phase is similar to that 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 NS causes an expansion of the Compton cloud similar to that found previously in Sco X-1 during the Flaring branch.