2006 May-July Major Radio Flare episodes in Cygnus X-3: spectro-timing analysis of the X-ray data

TL;DR

This study provides a detailed spectro-timing analysis of Cygnus X-3 during major radio flares in 2006, revealing the spectral components and their evolution, with implications for the jet shock region as the Comptonization site.

Contribution

It introduces a combined PCA and spectral fitting approach to dissect the X-ray spectral variability during flares, highlighting the roles of hybrid Comptonization and thermal components.

Findings

X-ray variability is driven by two principal spectral components.

Thermal component variability correlates with spectral state changes.

Jet shocks are likely sites of Comptonization during flares.

Abstract

We analyse in detail the X-ray data of the microquasar Cygnus X-3 obtained during major radio flaring episodes in 2006 with multiple observatories. The analysis consists of two parts: probing the fast (~ 1 minute) X-ray spectral evolution with Principal Component Analysis followed by subsequent spectral fits to the time-averaged spectra (~ 3 ks). Based on the analysis we find that the overall X-ray variability during major flaring episodes can be attributed to two principal components whose evolution based on spectral fits is best reproduced by a hybrid Comptonization component and a bremsstrahlung or saturated thermal Comptonization component. The variability of the thermal component is found to be linked to the change in the X-ray/radio spectral state. In addition, we find that the seed photons for the Comptonization originate in two seed photon populations that include the additional thermal emission and emission from the accretion disc. The Comptonization of the photons from the thermal component dominates, at least during the major radio flare episode in question, and the Comptonization of disc photons is intermittent and can be attributed to the phase interval 0.2-0.4. The most likely location for Comptonization is in the shocks in the jet.