X-ray Studies of the Extended TeV Gamma-Ray Source VER J2019+368

Abstract

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 $N(\mathrm{H})=(8.2_{-1.1}^{+1.3})\times 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$ 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 $10\mathrm{kpc}$ 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 ($N(\mathrm{H})=(7.5\pm 0.9)\times 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$ 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 $\sim 3\mu \mathrm{G}$ 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.

arXiv commentaccepted for publication in ApJ

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