Coronal Properties of Black Hole X-Ray Binaries in the Hard State as Seen by NuSTAR and Swift

Abstract

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 . 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 , from ∼5 × 10 38 down to as low as ∼5 × 10 32 . We confirm the existence of a “V”-shaped correlation between Γ and . Surprisingly, we observe a “Λ”-shaped correlation between and . The “cooler when brighter” branch in the high-luminosity regime ( ≳ 3 × 10 36 ) 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 – correlation) branch in the low-luminosity regime ( ≲ 3 × 10 36 ) is unexpected, thus it puts a new challenge to existing models of hot accretion flow/corona.