INTEGRAL-RXTE observations of Cygnus X-1

Abstract

We present first results from contemporaneous observations of Cygnus X-1 with INTEGRAL and RXTE, made during INTEGRAL'S performance verification phase in 2002 November and December. Consistent with earlier results, the 3-250 keV data are well described by Comptonization spectra from a Compton corona with a temperature of kT ∼ 50-90 keV and an optical depth of τ ∼ 1.0-1.3 plus reflection from a cold or mildly ionized slab with a covering factor of Ω/2π ∼ 0.2-0.3. A soft excess below 10 keV, interpreted as emission from the accretion disk, is seen to decrease during the 1.5 months spanned by our observations. Our results indicate a remarkable consistency among the independently calibrated detectors, with the remaining issues being mainly related to the flux calibration of INTEGRAL.