On the origin of the hard X-Ray excess of high-synchrotron-peaked BL Lac object Mrk 421

For the first time, Kataoka \& Stawarz reported a clear detection of a hard X-ray excess, above $\gtrsim$20 keV, in the high-synchrotron-peaked BL Lac object Mrk 421. We find that this feature may not be produced by the low-energy part of the same electron population that produced the Fermi/LAT $\gamma$-ray. Because of that it is required that the power-law electron energy go down to $\gamma_{\rm min}\approx19$, which predicts a very strong radio emission (radio flux larger than the observed) even considering the synchrotron self-absorption effect. We investigate the possibility of this excess being produced from the spine/layer jet structure, which has been clearly detected in Mrk 421. We find that (1) similar to one-zone modeling, the spine emissions provide good modeling of the broadband spectral energy distribution, except for the hard X-ray excess; and (2) the hard X-ray excess can be well represented by the synchrotron photons (from the layer) being inverse Compton scattered by the spine electrons.