  PSR~J2021+4026 showed a sudden decrease in the gamma-ray emission at the
glitch that occurred around 2011, October 16, and a relaxation of the flux to
the pre-glitch state at around 2014 December. We report X-ray analysis results
of the data observed by XMM-Newton on 2015 December 20 in the post-relaxation
state. To examine any change in the X-ray emission, we compare the properties
of the pulse profiles and spectra at the low gamma-ray flux state and at the
post-relaxation state. The phase-averaged spectra for both states can be well
described by a power-law component plus a blackbody component. The former is
dominated by unpulsed emission and is probably originated from the pulsar wind
nebula as reported by Hui et al (2015). The emission property of the blackbody
component is consistent with the emission from the polar cap heated by the
back-flow bombardment of the high-energy electrons or positrons that were
accelerated in the magnetosphere. We found no significant change in the X-ray
emission properties between two states. We suggest that the change of the X-ray
luminosity is at an order of ~4%, which is difficult to measure with the
current observations. We model the observed X-ray light curve with the heated
polar cap emission and we speculate that the observed large pulsed fraction is
owing to asymmetric magnetospheric structure.
