  We report the discovery of X-ray periodicity of ~265.3 ms from a deep
XMM-Newton observation of the radio-quiet gamma-ray pulsar, PSR J2021+4026,
located at the edge of the supernova remnant G78.2+2.1 (gamma-Cygni).The
detected frequency is consistent with the gamma-ray pulsation determined by the
observation of Fermi Gamma-ray Space Telescope at the same epoch. The X-ray
pulse profile resembles the modulation of hot spot on the surface of the
neutron star. The phase-averaged spectral analysis also suggests that the
majority of the observed X-rays have a thermal origin. This is the third member
in the class of radio-quiet pulsars with the significant pulsations detected
from both X-rays and gamma-ray regimes.
