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 be modeled as a soft power-law component, with the photon index of ≃2.4, on top of thermal Comptonization emission from a plasma with electron temperature kTe ≃ 6 keV and optical depth τ ≃ 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 ≳4 keV spectra are described by thermal Comptonization with kTe ≃ 3 keV and τ ≃ 6-7.