We detected X-ray emission from the 50-kyr-old pulsar J1809–1917 and resolved its pulsar wind nebula ( PWN ) with the Chandra X-ray Observatory .
The pulsar ’ s observed flux is F _ { psr } = ( 1.8 \pm 0.2 ) \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } in the 1–6 keV band .
A two-component blackbody+power-law ( BB+PL ) fit of the pulsar ’ s spectrum yields the photon index \Gamma _ { psr } = 1.2 \pm 0.6 and luminosity L _ { psr } = ( 4 \pm 1 ) \times 10 ^ { 31 } ergs s ^ { -1 } of the PL component , in the 0.5–8 keV band , for a plausible distance d = 3.5 kpc and n _ { H } = 0.7 \times 10 ^ { 22 } cm ^ { -2 } .
The BB component corresponds to the temperature T \approx 2 MK , and bolometric luminosity L _ { bol } \sim 1 \times 10 ^ { 32 } ergs s ^ { -1 } .
The bright inner PWN component of a 3 ^ { \prime \prime } \times 12 ^ { \prime \prime } size is elongated in the north-south direction , with the pulsar close to its south end .
This component is immersed in a larger ( \approx 20 ^ { \prime \prime } \times 40 ^ { \prime \prime } ) , similarly elongated outer PWN component of lower surface brightness .
The elongated shape of the compact PWN can be explained by the ram pressure confinement of the pulsar wind due to the supersonic motion of the pulsar .
The observed flux of the compact PWN , including both components , is F _ { pwn } \simeq ( 1.5 \pm 0.1 ) \times 10 ^ { -13 } ergs cm ^ { -2 } s ^ { -1 } in the 1–6 keV band .
The PWN spectrum can be fitted with a PL model with n _ { H } \approx 0.7 \times 10 ^ { 22 } cm ^ { -2 } and photon index \Gamma _ { pwn } = 1.4 \pm 0.1 , corresponding to the 0.5–8 keV luminosity L _ { pwn } \approx 4 \times 10 ^ { 32 } ergs s ^ { -1 } .
The compact PWN appears to be inside a more extended ( \approx 4 ^ { \prime } \times 4 ^ { \prime } ) emission with the total observed flux F _ { ext } \sim 5 \times 10 ^ { -13 } ergs s ^ { -1 } in the 0.8–7 keV band .
This large-scale emission is more extended to the south of the pulsar , i.e .
in the direction of the alleged pulsar motion .
To explain the extended X-ray emission ahead of the moving pulsar , one has to invoke strong intrinsic anisotropy of the pulsar wind or assume that this emission comes from a relic PWN swept by the asymmetrical reverse SNR shock .
The pulsar and its PWN are located within the extent of the unidentified TeV source HESS J1809–193 .
The brightest part of the TeV source is offset by \sim 8 ^ { \prime } to the south of the pulsar , i.e .
in the same direction as the large-scale X-ray emission .
Although the association between the PSR J1809–1917 and HESS J1809–193 is plausible , an alternative source of relativistic electrons powering HESS J1809–193 might be the serendipitously discovered X-ray source CXOU J180940.7 - 192544 .
In addition to the CMBR or Galactic starlight background , the low-frequency seed photons for Compton upscattering to TeV energies might be supplied by bright infrared emission from dust-molecular clouds seen within HESS J1809–193 .