Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram

TL;DR

This study examines the broadband spectral evolution of Scorpius X-1 across different accretion states using RXTE data, revealing the need for a complex spectral model including a power-law component at high energies.

Contribution

It introduces a detailed spectral analysis of Scorpius X-1, showing that a simple two-component model is insufficient at low accretion rates and proposing a more complex model with a power-law component.

Findings

A two-component model fits most spectra but fails at high energies.

A third component, modeled as a power-law, is necessary at high energies.

Results suggest complex physical processes in the X-ray emission regions.

Abstract

We analyze a large collection of RXTE archive data from April 1997 to August 2003 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 studying energy spectra from selected regions in the Z-track of its Color-Color Diagram. A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3--200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a power-law component, that could represent a second thermal Comptonization from a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. 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 origins.