Hard X‐Ray Emission of the Microquasar GX 339−4 in the Low/Hard State

Abstract

We present the analysis of the high-energy emission of the Galacticblack hole binary GX 339-4 in a low/hard state at the beginning of its2004 outburst. The data from 273 ks of INTEGRAL observations, spreadover 4 weeks, are analyzed, along with the existing simultaneous RXTEHEXTE and PCA data. During this period, the flux increases by a factorof similar or equal to 3, while the spectral shape is quite unchanged,at least up to 150 keV. The high-energy data allow us to detect thepresence of a high-energy cutoff, generally related to thermalmechanisms, and to estimate the plasma parameters in the framework ofthe Comptonization models. We found an electron temperature of 60-70 keVand an optical depth of around 2.5, with a rather low reflection factor(0.2-0.4). In the last observation, we detected a high-energy excessabove 200 keV with respect to thermal Comptonization, while at lowerenergies the spectrum is practically identical to the previous one takenjust 2 days before. This suggests that the low- and high-energycomponents have a different origin.