  To understand the origin of hard X-ray emissions from black hole X-ray
binaries during their low/hard states, we calculate the X-ray spectra of
black-hole accretion flow for the following three configurations of hot and
cool media: (a) an inner hot flow and a cool outer disk (inner hot flow model),
(b) a cool disk sandwiched by disk coronae (disk corona model), and (c) the
combination of those two (hybrid model). The basic features we require for
successful models are (i) significant hard X-ray emission whose luminosity
exceeds that of soft X-rays, (ii) high hard X-ray luminosities in the range of
(0.4 - 30) times 10^{37} erg s^{-1}, and (iii) the existence of two power-law
components in the hard X-ray band with the photon indices of Gamma_s ~ 2 >
Gamma_h, where Gamma_s and Gamma_h are the photon indices of the softer (<10
keV) and the harder (>10 keV) power-law components, respectively. Contribution
by non-thermal electrons nor time-dependent evolution are not considered. We
find that Models (a) and (b) can be ruled out, since the spectra are always
dominated by the soft component, and since only one power-law component, at
most, can be reproduced. Only Model (c) can account for sufficiently strong
hard X-ray emissions, as well as the existence of the two power-law components,
for a large ratio of the accretion rate in the corona to that in the thin disk.
The outer disk corona (where the Compton y-parameter is smaller, y < 1)
produces the softer power-law component with photon index of Gamma_s ~ 2,
whereas the inner hot flow (where y gtrsim 1) generates the harder component
with Gamma_h < 2. This model can also account for the observed relationship
between the photon index and the reflection fraction.
