The superb spatial resolution of Chandra has allowed us to detect a 20 ^ { \prime \prime } -long tail behind the Geminga pulsar , with a hard spectrum ( photon index \Gamma = 1.0 \pm 0.2 ) and a luminosity ( 1.3 \pm 0.2 ) \times 10 ^ { 29 } ergs s ^ { -1 } in the 0.5–8 keV band , for an assumed distance of 200 pc .
The tail could be either a pulsar jet , confined by a toroidal magnetic field of \sim 100 \mu G , or it can be associated with the shocked relativistic wind behind the supersonically moving pulsar confined by the ram pressure of the oncoming interstellar medium .
We also detected an arc-like structure 5 ^ { \prime \prime } – 7 ^ { \prime \prime } ahead of the pulsar , extended perpendicular to the tail , with a factor of 3 lower luminosity .
We see a 3 \sigma enhancement in the Chandra image apparently connecting the arc with the southern outer tail that has been possibly detected with XMM-Newton .
The observed structures imply that the Geminga ’ s pulsar wind is intrinsically anisotropic .