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

Abstract

Context. 4U 1705-44 is one of the best-studied type I X-ray burster and atoll sources. Aims. 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 for testing the accretion models proposed for atoll sources. Methods. We analyzed the energy spectrum accumulated with BeppoSAX observations (43.5 ks) in August 2000 when the source was in a soft spectral state. Results. The continuum of the wide-band energy spectrum is well-described by the sum of a blackbody (kT(bb) similar to 0.56 keV) and a Comptonized component (seed-photon temperature kT(W) similar to 1 keV, electron temperature kT(e) similar to 2.7 keV, and optical depth tau similar to 11). A hard tail was detected at energies above similar to 25 keV. The latter can be modeled by a power law having a photon index similar to 2.9, which contributes similar to 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. Conclusions. This is the first time that a high-energy tail has been observed during a soft state of the source.