  We propose a modified version of the X-ray spectral index and an intrinsic
cutoff frequency of inverse Compton radiation from the brightest knot of the
M87 jet, in conjunction with an application of the new conceptions of injection
and diffusive shock acceleration (DSA) of electrons in magnetized filamentary
plasma to the specified source. The drop of the X-ray flux density in a
transitive frequency region is associated with the interplay of ordinary
synchrotron cooling and weaker magnetic fields concomitant with the smaller
scale filaments that allow the electron injection, while the radio-optical
synchrotron continuum is dominantly established by the major electrons that are
quasi-secularly bound to larger filaments. With reference to, particularly, the
updated external Compton model, we demonstrate that in the Klein-Nishina regime
fading inverse Comptonization, the injected electrons can be stochastically
energized up to a Lorentz factor as high as $5\times 10^{10}$ in the temporal
competition with diffuse synchrotron cooling; this value is larger than that
attainable for a simple DSA scenario based on the resonant scattering diffusion
of the gyrating electrons bound to a supposed magnetic field homogeneously
pervading the entire knot. The upper limits of the photon frequency boosted via
conceivable inverse Compton processes are predicted to be of the common order
of $\sim 10^{30}$ Hz. The variability of the broadband spectrum is also
discussed in comparison to the features of a blazar light curve. The present
scenario of a peta-eV (PeV; $10^{15}$ eV) electron accelerator, the "Pevatron,"
might provide some guidance for exploring untrod hard X-ray and gamma-ray bands
in forthcoming observations.
