This article reports the results of X-ray studies of the extended TeV \gamma -ray source VER J2019+368 . Suzaku observations conducted to examine properties of the X-ray pulsar wind nebula ( PWN ) around PSR J2021+3651 revealed that the western region of the X-ray PWN has a source extent of 15 ^ { \prime } \times 10 ^ { \prime } with the major axis oriented to that of the TeV emission .
The PWN-west spectrum was closely fitted by a power-law for absorption at N ( { H } ) = ( 8.2 ^ { +1.3 } _ { -1.1 } ) \times 10 ^ { 21 } ~ { } { cm ^ { -2 } } and a photon index of \Gamma = 2.05 \pm 0.12 , with no obvious change in the index within the X-ray PWN .
The measured X-ray absorption indicates that the distance to the source is much less than 10 ~ { } { kpc } inferred by radio data .
Aside from the PWN , no extended emission was observed around PSR J2021+3651 even by Suzaku .
Archival data from the XMM-Newton were also analyzed to complement the Suzaku observations , indicating that the eastern region of the X-ray PWN has a similar spectrum ( N ( H ) = ( 7.5 \pm 0.9 ) \times 10 ^ { 21 } ~ { } { cm ^ { -2 } } and \Gamma = 2.03 \pm 0.10 ) and source extent up to at least 12 ^ { \prime } along the major axis .
The lack of significant change in the photon index and the source extent in X-ray are used to constrain the advection velocity or the diffusion coefficient for accelerated X-ray-producing electrons .
A mean magnetic field of { \sim } 3 ~ { } \mu { G } is required to account for the measured X-ray spectrum and reported TeV \gamma -ray spectrum .
A model calculation of synchrotron radiation and inverse Compton scattering was able to explain { \sim } 80 \% of the reported TeV flux , indicating that the X-ray PWN is a major contributor of VER J2019+368 .