On the spectral evolution of Cygnus X-2 along its color-color diagram

We report on the results of a broad band (0.1-200 keV) spectral study of Cyg X-2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of similar to100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around different positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are fit to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at similar to1 keV and similar to6.6 keV, respectively. The addition of a hard power-law tail with photon index similar to2, contributing similar to1.5% of the source luminosity, improves the fit of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, kT(in), varying between similar to0.8 and similar to1.7 keV and inner radius, R-in, varying between similar to26 and similar to11 km (assuming an inclination angle of the system of 60degrees). The Comptonization component is probably emitted by hot plasma (electron temperature kT(e) varying between similar to3 and similar to20 keV, optical depth tau similar to 11-0.4, seed-photon temperature kT(W) similar to 1-2.4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change significantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.