The role of comptonisation in energy spectra of standard QPO-sources

The spectral variability of Quasi-Periodic Spectral (QPO) sources is analyzed from EXOSAT data. It is found that blackbody comptonized by hot electron plasma fit the EXOSAT spectra of these sources. The fit is as accurate as using two component spectral models. This equivalency is observed within all spectral branches. A parametrization of the color-color diagram indicates systematical parameter variations in all spectral branches. They can be decomposed in a variation of single photon source and an extended hot electron cloud. The temperature of the photon source varies between 0.5 and 1.0 keV, the electron cloud has scattering depths between 3 and 7 and temperatures from 2.0 to 5.0 keV. Links to QPO-models on the horizontal spectral branch and physical consequences of the model are discussed.