2002A&A...386..535D

We report on the results of a broad band (0.1-200keV) spectral study of Cyg X-2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of ∼100ks. 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 ∼1keV and ∼6.6keV, respectively. The addition of a hard power-law tail with photon index ∼2, contributing ∼1.5% 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 ∼0.8 and ∼1.7keV and inner radius, R_in, varying between ∼26 and ∼11km (assuming an inclination angle of the system of 60°). The Comptonization component is probably emitted by hot plasma (electron temperature kT_e varying between ∼3 and ∼20keV, optical depth τ∼11-0.4, seed-photon temperature kT_W∼1-2.4keV) 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.