2007A&A...471L..17P

U 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 50x10³⁶erg/s) and shows clear spectral state transitions, it represents a good laboratory for testing the accretion models proposed for atoll sources. We analyzed the energy spectrum accumulated with BeppoSAX observations (43.5ks) 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∼0.56keV) and a Comptonized component (seed-photon temperature kT_W∼1keV, electron temperature kT_e∼2.7keV, and optical depth τ∼11). A hard tail was detected at energies above ∼25keV. 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 0.1-200keV. 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.