  Pulsar wind nebulae (PWNe) are one of the most energetic galactic sources
with bright emissions from radio waves to very high-energy gamma-rays. We
perform wideband X-ray spectroscopy of four energetic PWNe, N157B, PSR
J1813-1749, PSR J1400-6325, and G21.5-0.9, with the Suzaku, Chandra, NuSTAR,
and Hitomi observatories. A significant spectral break or cutoff feature is
found in the hard X-ray band for all the samples, except for N157B. The break
energies in the broken power-law fitting are in the range of 4--14 keV, whereas
the cutoff energies in the cutoff power-law fitting are at 22 keV or higher.
The break or cutoff energy does not show a significant correlation with either
the spin-down energy or characteristic age of the hosting pulsars. A possible
correlation is found between the photon index change in the broken power-law
fitting and the X-ray emitting efficiency of the pulsars, although its
significance is not high enough to be conclusive. We discuss what determines
the break parameters based on simple models.
