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

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 × 10³⁶ 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∼ 0.56 keV) and a Comptonized component (seed-photon temperature kT_W ∼ 1 keV, electron temperature kT_e∼ 2.7 keV, and optical depth τ∼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.
Conclusions: This is the first time that a high-energy tail has been observed during a soft state of the source.