2020ApJ...889L..18Y

In this work we measure two important phenomenological parameters of corona (and hot accretion flow) in black hole X-ray binaries: the photon index Γ and the electron temperature kT_e. Thanks to the capability of the Nuclear Spectroscopic Telescope Array in hard X-rays, we measure these two parameters over six orders of magnitude in the 0.1-100 keV X-ray luminosity L_x, from ∼5 x 10³⁸ ergs^–1 down to as low as ∼5 x 10³² ergs^–1. We confirm the existence of a "V"-shaped correlation between Γ and L_x. Surprisingly, we observe a "Λ"-shaped correlation between kT_e and L_x. The "cooler when brighter" branch in the high-luminosity regime (L_x >= 3 x 10³⁶ ergs^–1) agrees with previous results and can be understood under the existing model of Compton scattering in the corona. On the other hand, the apparent "cooler when fainter" (positive kT_e-L_x correlation) branch in the low-luminosity regime (L_x <= 3 x 10³⁶ ergs^–1) is unexpected, thus it puts a new challenge to existing models of hot accretion flow/corona.