Accretion Disk Structure and Branch Behavior of Cygnus X-2

A new analysis of the branch behavior of Cygnus X-2 is presented, applying a new disk model with a free parameter of a radial temperature gradient, q = - d log T/d log r, for the spectral fitting. We find that the spectral fitting is significantly improved for about one third of data and that the derived q-value changes from 0.67 to 0.85 in accordance with the spectral evolution from the horizontal branch (HB) to the flaring branch (FB) via the normal branch (NB). Two positive and negative correlations are found between the inner disk radius (R_in) and q, and between R_in and the inner disk temperature (T_in) Possible new interpretations regarding the branch behavior of Cyg X-2 are discussed based on these results.