Luminosity dependence of the electron temperature in the bright hard state of the black hole candidate GX 339--4

Abstract

We have analyzed 200 Rossi X-ray Timing Explorer observations of the black hole candidate GX 339--4, all from the bright hard state periods between 1996 and 2005. Purpose of our study is to investigate the radiation mechanisms in the hard state of GX 339--4. The broadband 3--200 keV spectra were successfully modeled by a simple analytic model, power--law with an exponential cut-off modified with a smeared edge. The obtained energy cut-off ($E_{\mathrm{cut}}$) was distributed over 50--200 keV, and the photon index over 1.4--1.7. We found a clear anti-correlation ($E_{\mathrm{cut}}\propto L^{-0.70\pm 0.06}$) between the X-ray luminosity ($L$) in 2--200 keV and $E_{\mathrm{cut}}$, when $L$ is larger than $7\times 10^{37}$ erg s${}^{-1}$ (assuming a distance of 8 kpc), while $E_{\mathrm{cut}}$ is roughly constant at around 200 keV when $L$ is smaller than $7\times 10^{37}$ erg s${}^{-1}$. This anti-correlation remained unchanged by adopting a more physical thermal Comptonization model, which resulted in the anti-correlation that can be expressed as $k{T}_{\mathrm{e}}\propto L^{-0.24\pm 0.06}$. These anti-correlations can be quantitatively explained by a picture in which the energy-flow rate from protons to electrons balances with the inverse Compton cooling.

Accepted by PASJ. 15 pages, 8 figures

References in corpus (3)

• INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4
• Hard X-ray emission of the microquasar GX 339-4 in the low/hard state
• The high energy emission of GRO J1655-40 as revealed with INTEGRAL spectroscopy of the 2005 outburst

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