INTEGRAL spectral variability study of the atoll 4U 1820-30: First detection of hard X-ray emission

We study the 4-200 keV spectral and temporal behavior of the low-mass X-ray binary 4U 1820 - 30 with INTEGRAL during 2003-2005. This source as been observed in both the soft ( banana) and hard ( island) spectral states. A high-energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can bemodeled as a soft power-law component, with the photon index of similar or equal to 2.4, on top of thermal Comptonization emission from a plasma with electron temperature kT(e) similar or equal to 6 keV and optical depth tau similar or equal to 4. Alternatively, but at a poorer goodness of fit, the hard-state broadband spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the greater than or similar to 4 keV spectra are described by thermal Comptonization with kT(e) similar or equal to 3 keV and tau similar or equal to 6-7.