BeppoSax and RXTE spectral study of the low-mass X-ray binary 4U 1705–44: Spectral hardening during the banana branch
- Moscow State University/Sternberg Astronomical Institute, Universitetsky Prospect 13, Moscow, 119992 (Russian Federation)
- Dipartimento di Fisica, Università di Ferrara, Via Saragat 1, I-44122 Ferrara (Italy)
- NASA Goddard Space Flight Center, NASA, Astrophysics Science Division, Code 661, Greenbelt, MD 20771 (United States)
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{sub s1}≲1.5 keV (herein Comptb1), and a second results from the seed photons of T{sub 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 Γ{sub 1} of our model is always about 2, independently of the spectral state. Another parameter, Γ{sub 2}, demonstrates a two-phase behavior depending on the spectral state. Γ{sub 2} is quasi-constant at Γ{sub 2}∼2 when the electron temperature kT{sub e}{sup (2)}<80 keV, and Γ{sub 2} is less than 2, in the range of 1.3<Γ{sub 2}<2, when kT{sub e}{sup (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.
- OSTI ID:
- 22882835
- Journal Information:
- Astrophysical Journal, Vol. 808, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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