  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 ~100 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 ~1 keV and ~6.6 keV, 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, k T_in, varying between 0.8 and 1.7 keV and inner radius, R_in,
varying between 26 and 11 km (assuming an inclination angle of the system of 60
degrees). The Comptonization component is probably emitted by hot plasma
(electron temperature k T_e varying between 3 and ~20 keV, optical depth \tau ~
11-0.4, seed-photon temperature k T_W ~ 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.
