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

TL;DR

This study analyzes the spectral states of the low-mass X-ray binary 4U 1705-44 using RXTE and BeppoSAX data, revealing spectral hardening during the banana branch and identifying a two-component Comptonization model with state-dependent photon index behavior.

Contribution

It introduces a two-Comptonization component spectral model for 4U 1705-44 and characterizes the state-dependent behavior of the photon index Gamma_2, linking it to radiation pressure effects.

Findings

Photon index Gamma_1 remains constant at ~2 across states.

Gamma_2 shows a two-phase behavior depending on electron temperature.

Spectral hardening correlates with super-Eddington radiation pressure effects.

Abstract

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 kT_{s1}<1.5 keV (herein Comptb) and a second resulting 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 Gamma_{1} of our model is always about 2, independently of the spectral state. Another parameter, Gamma_{2} demonstrates a two-phase behavior depending on the spectral state. Gamma_{2} is quasi-constant at Gamma_{2}~ 2 when the electron temperature kT^{(2)}_e<80 keV and Gamma_{2} is less than 2, in the range of 1.3<Gamma_{2}<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.