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

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 10^{36} 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 (kT_{bb}~0.56 keV) and a Comptonized component (seed-photon temperature kT_W~1 keV, electron temperature kT_e~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 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.