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

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.

19 pages, 11 figures, 5 Tables, Accepted for publication in The Astrophysical Journal

References in corpus (8)

• Average hard X-ray emission from NS LMXBs: Observational evidence of different spectral states in NS LMXBs
• A relativistically smeared spectrum in the neutron star X-ray Binary 4U 1705-44: Looking at the inner accretion disc with X-ray spectroscopy
• Black hole mass determination in the X-ray binary 4U 1630--47: Scaling of spectral and variability characteristics
• Suzaku broad-band spectrum of 4U 1705-44: Probing the Reflection component in the hard state
• BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state
• Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• The first detection of Compton Reflection in the Low-Mass X-ray Binary 4U1705-44 with INTEGRAL and BeppoSAX

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