Simultaneous ASCA and RXTE observations of Cygnus X-1 during its 1996 state transition

We report results from simultaneous ASCA and RXTE observations of Cygnus X-1 when the source made a rare transition from the hard (=low) state to the soft (=high) state in 1996. These observations together cover a broad energy range similar to 0.7-50 keV with a moderate energy resolution at the iron K band and thus make it possible to disentangle various spectral components. The low-energy spectrum is dominated by an ultrasoft component, which is likely to be the emission from the hottest inner portion of the accretion disk around the black hole. At high energies, the X-ray spectrum can be described by a Comptonized spectrum with a reflection component. The Compton corona, which upscatters soft "seed photons" to produce the hard X-ray emission, is found to have a y-parameter similar to 0.28. The hard X-ray emission illuminates the accretion disk, and the reemitted photons produce the observed "reflection bump." We show that the reflecting medium subtends only a small solid angle (similar to 0.15 x 2 pi but has a large ionization parameter such that iron is ionized up to Fe XXIV-Fe XXVI. The presence of a broad iran line at 6.58 +/- 0.04 keV is also consistent with a highly ionized disk, if we take into account the gravitational and Doppler shift of the line energy. These results imply a geometry of a central corona surrounding the black hole and the reflection occurring in the innermost region of the disk where matter is highly ionized.