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Preprint 10/07

BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state

S. Piraino (1,2), A. Santangelo (2), T. Di Salvo (3), P. Kaaret (4), D. Horns (2), R. Iaria (3), L Burderi (5)

(1) INAF-ISAF di Palermio, Via Ugo La Malfa 153, 90146 Palermo, Italy
(2) IAAT, University of Tübingen, Sand 1 72076 Tübingen, Germany
(3) Dipartimento di Scienze Fisiche ed Astronomiche, Universita di Palermo, via Archirafi 36 90123 Palermo, Italy
(4) Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52246, USA
(5) Universita degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu KM 0.7, 09042 Monserrato, Italy

To be published in: A&A Letters

Abstract. 4U 1705-44 is one of the best studied type I X-ray burster and atoll sources. Since it covers a wide range in luminosity (from a few to 50 x 1036 erg s-1) and shows clear spectral state transitions, it represents a good laboratory to test the accretion models proposed for atoll sources. We analysed the energy spectrum accumulated with BeppoSAX observations (43.5 ksec) in August 2000 when the source was in a soft spectral state. The continuum of the wide band energy spectrum is well described by the sum of a blackbody (kTbb~0.56 keV) and a Comptonized component (seed-photon temperature kTW~1 keV, electron temperature kTe~2.7 keV, and optical depth ~11). A hard tail was detected at energies above ~25 keV. The latter can be modeled by a power law having a photon index ~2.9 which contributes ~11% of the total flux in the range 0.1-200 keV. A broad emission line, possibly from a relativistic accretion disk, models the feature in the Fe K&nsbp;line region of the spectrum. This is the first time that a high energy tail has been observed during a soft state of the source.

Astrophysics (astro-ph): 0706.2784

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