Spectral Evolution of Scorpio X-1 along its Color-Color Diagram

Abstract

We analyze a large collection of RXTE archive data of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by selecting energy spectra from its Color-Color Diagram. We model the spectra with the combination of two absorbed components: a soft thermal component, which can be interpreted as thermal emission from an accretion disk, and a hybrid Comptonization component, which self-consistently includes the Fe Kα fluorescence line and the Compton reflected continuum. The presence of hard emission in Scorpius X-1 has been previously reported, however, without a clear relation with the accretion rate. We show, for the first time, that there exists a common trend in the spectral evolution of the source, where the spectral parameters change in correlation with the position of the source in the CD. Using a hybrid thermal/non-thermal Comptonization model (EQPAIR code), we show that the ratio of the power supplied to the non-thermal distribution to the total power injected into the Comptonizing plasma correlates with the accretion rate, being the highest at the lowest accretion rates. We discuss the physical implications derived from the results of our analysis, with a particular emphasis on the hardest part of the X-ray emission and its possible origin.