  The observed nonthermal afterglow spectrum of the binary neutron star (BNS)
merger GW170817 from radio to X-ray are consistent with synchrotron radiation
by shock-accelerated electrons. However, previous afterglow modeling studies
were based on a simplified assumption that the acceleration efficiency is
extremely high, i.e. all electrons in the shock are accelerated as a nonthermal
population. This affects the estimate of the minimum electron energy and hence
$\nu_m$, the peak frequency of the afterglow spectrum. Here we present Bayesian
fitting to the observed data with a more natural electron energy distribution,
in which the acceleration efficiency is a free parameter. Interestingly, the
maximum likelihood solutions are found with radio flux below $\nu_m$ in the
early phase, in contrast to previous studies that found the radio frequency
always above $\nu_m$. Therefore the $\nu_m$ passage through the radio band
could have been clearly detected for GW170817, if sufficient low-frequency
radio data had been taken in early time. In the new solutions, the lowest
energy of electrons is found close to equipartition with the post shock
protons, but only a small fraction ($<$10\%) of electrons are accelerated as
nonthermal particles. The jet energy and interstellar medium density are
increased by 1--2 orders of magnitude from the conventional modeling, though
these are still consistent with other constraints. We encourage to take densely
sampled low-frequency radio data in the early phase for future BNS merger
events, which would potentially detect $\nu_m$ passage and give a strong
constraint on electron energy distribution and particle acceleration
efficiency.
