GRS 1915+105: high-energy insights with SPI/INTEGRAL. Spectral analysis of the Comptonized emission

Abstract

Context: We report on results of nearly two years of INTEGRAL/SPI monitoring of the Galactic microquasar GRS 1915+105.
Aims: From September 2004 to May 2006, the source was observed twenty times with long (~100 ks) exposures. We present an analysis of the SPI data and focus on the description of the high-energy (>20 keV) output of the source.
Methods: We performed temporal and spectral analysis of the SPI data and considered simultaneous 1.2-12 keV ASM data.
Results: We found that the 20-500 keV spectral emission of GRS 1915+105 was bound between two states. It seems that these high-energy states are not correlated with the temporal behavior of the source, suggesting that there is no direct link between the macroscopic characteristics of the coronal plasma and the variability of the accretion flow. All spectra are well-fitted by a thermal Comptonization component plus an extra high-energy power law. This confirms the presence of thermal and non-thermal electrons around the black hole.