First IBIS results on the high energy emission of Cygnus X-2

The bright low-mass X-ray binary Cyg X-2 was in the field of view of the IBIS telescope during the early Cygnus region observations, executed during the INTEGRAL Performance Verification Phase. The data presented are spanning about one week and cover the rising edge of one of the two peaks of the ≈ 82 day cycle of the Cyg X-2 light curve. The IBIS data in the energy range 20-40 keV exhibit flux variation correlated with the RXTE /ASM light curve. Two different main exposures, separated by ≈ 5 days are found to be characterized by sensibly different spectra, with significant softening and higher X-ray luminosity in the second part, coincident with the long-term cycle peak. At high energies, both measured spectra are very steep. The ratio of the 30-45 keV and 20-30 keV detected fluxes is ≈ 0.30, against a value of 0.95 expected for a Crab-like spectrum. No positive detection exists for $E\geq45$ keV, with a flux upper limit ($5\sigma$) of ~$1.4\times10^{-10}$ erg cm -2 s -1 in the 45-100 keV band.