  The shock waves of supernova remnants (SNRs) are prominent candidates for the
acceleration of the Galactic cosmic rays. SNR RX J1713.7$-$3946 is one
well-studied particle accelerator in our Galaxy because of its strong
non-thermal X-ray and gamma-ray radiation. We have performed NuSTAR (3-79 keV)
observations of the northwest rim of RX J1713.7$-$3946, where is the brightest
part in X-ray and the shock speed is about 4000 km s$^{-1}$. The spatially
resolved X-ray emission from RX J1713.7$-$3946 is detected up to 20 keV for the
first time. The hard X-ray image in 10-20 keV is broadly similar to the
soft-band image in 3-10 keV. The typical spectrum is described by power-law
model with exponential cutoff with the photon index $\Gamma$=2.15 and the
cutoff energy $\varepsilon_c$=18.8 keV. Using a synchrotron radiation model
from accelerated electrons in the loss-limited case, the cutoff energy
parameter ranges 0.6-1.9 keV, varying from region to region. Combined with the
previous measurement of the shock speed, the acceleration of electrons is close
to the Bohm-limit regime in the outer edge, while the standard picture of
accelerated particles limited by synchrotron radiation in SNR shock is not
applicable in the inner edge and the filamentary structure.
