Spectral break of energetic pulsar wind nebulae detected with wideband X-ray observations

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, N 157 B, PSR J1813-1749, PSR J1400-6325, and G21.5-0.9, with the Suzaku, Chandra, NuSTAR, and Hitomi observatories. A significant spectral break or cut-off feature is found in the hard X-ray band for all the samples, except for N 157 B. The break energies in the broken power-law fitting are in the range of 4-14 keV, whereas the cut-off energies in the cut-off power-law fitting are at 22 keV or higher. The break or cut-off 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.