The X-ray spectral evolution of Cyg X–2 in the framework of bulk Comptonization

Context. The interplay of thermal and bulk motion Comptonization to explain the spectral evolution of neutron star LMXBs including transient hard X-ray tails is gaining a strong theoretical and observational support. The last momentum has been given by the advent of a new XSPEC Comptonization model, COMPTB, which includes thermal and bulk Comptonization. Aims. We used COMPTB to investigate the spectral evolution of the neutron star LMXB Cyg X–2 along its Z-track. We selected a single source in order to trace in a quantitative way the evolution of the physical parameters of the model. Methods. We analyzed archival broad-band BeppoSAX spectra of Cyg X–2. Five broad-band spectra have been newly extracted according to the source position in the Z-track described in the color-color and color-intensity diagrams. Results. We have fitted the spectra of the source with two COMPTB components. The first one, with bulk parameter δ=0, dominates the overall source broad-band spectrum and its origin is related to thermal upscattering (Comptonization) of cold seed photons off warm electrons in high-opacity enviroment. We attribute the origin of these seed photons to the part of the disk which illuminates the outer coronal region (transition layer) located between the accretion disk itself and the neutron star surface. This thermal component is roughly constant with time and with inferred mass accretion rate. The second COMPTB model describes the overall Comptonization (thermal plus bulk, δ > 0) of hotter seed photons which come from both the inner transition layer and from the neutron star surface. The appearance of this component in the colour-colour or hardness-intensity diagram is more pronounced in the horizontal branch and is progressively disappearing towards the normal branch, where a pure blackbody spectrum is observed. Conclusions. The spectral evolution of Cyg X–2 is studied and interpreted in terms of changes in the innermost environmental conditions of the system, leading to a variable thermal-bulk Comptonization efficiency.