Clear anti-correlation between luminosity and high energy cutoff in the low/hard state of the black hole candidate GX339-4

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($E_{\mathrm{c}\mathrm{u}\mathrm{t}}$) is distributed over 50–300 keV, and the photon index over 1.4-1.6. We found a clear correlation ($E_{\mathrm{c}\mathrm{u}\mathrm{t}}$ is proportional to $L^{-0.75\pm 0.04}$) between luminosity in 2-200 keV (L) and $E_{\mathrm{c}\mathrm{u}\mathrm{t}}$ when L is larger than 5$\times 10^{37}$erg$s^{-1}$,while$E_{\rm{cut}}$ 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.