We have analyzed the 171 RXTE data sets of the black hole candidate GX 339-4 in the low/hard state during its 1996–2005 outburst .
All the broadband spectra were successfully modeled by a simple analytic model , power-law with an exponential cutoff .
The obtained energy cutoff ( E _ { cut } ) is distributed over 50–300 keV , and the photon index over 1.4–1.6 .
We found a clear correlation ( E _ { cut } is proportional to L ^ { -0.75 \pm 0.04 } ) between luminosity in 2–200 keV ( L ) and E _ { cut } when L is larger than 5 \times 10 ^ { 37 } erg s ^ { -1 } , while E _ { cut } is almost constant at 200 keV when L is smaller than 5 \times 10 ^ { 37 } erg s ^ { -1 } .
This anti-correlation is unchanged by adopting the more physical model of thermal Comptonization by Sunyaev and Titarchuk , although a slightly different relation is obtained as the electron temperature is proportional to L ^ { -0.23 \pm 0.02 } .
These anti-correlations are qualitatively explained by a picture where the energy flow rate from protons to electrons balance with cooling due to inverse Compton scattering .