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

We present the analysis of the high energy emission of the Galactic blackhole binary GX 339-4 in a low/hard state at the beginning of its 2004 outburst.The data from 273 ks of INTEGRAL observations, spread over 4 weeks, areanalyzed, along with the existing simultaneous RXTE HEXTE and PCA data. Duringthis period, the flux increases by a factor of about 3, while the spectralshape is quite unchanged, at least up to 150 keV. The high energy data allowsus to detect the presence of a high energy cut-off, generally related tothermal mechanisms, and to estimate the plasma parameters in the framework ofthe Comptonization models. We found an electron temperature of 60-70 keV, anoptical depth around 2.5, with a rather low reflection factor (0.2-0.4). In thelast observation, we detected a high energy excess above 200 keV with respectto thermal Comptonization while at lower energy, the spectrum is practicallyidentical to the previous one, taken just 2 days before. This suggests that thelow and high energy components have a different origin.Comment: 12 pages, 6 figures, accepted for publication in Ap