PSR J0357+3205: A FAST-MOVING PULSAR WITH A VERY UNUSUAL X-RAY TRAIL

The middle-aged PSR J0357+3205 is a nearby, radio-quiet, bright gamma-ray pulsar discovered by the Fermi mission. Our previous Chandra observation revealed a huge, very peculiar structure of diffuse X-ray emission originating at the pulsar position and extending for >9' on the plane of the sky. To better understand the nature of such a nebula, we have studied the proper motion of the parent pulsar. We performed relative astrometry on Chandra images of the field spanning a time baseline of 2.2 yr, unveiling a significant angular displacement of the pulsar counterpart, corresponding to a proper motion of 0.'' 165 +/- 0.'' 030 yr(-1) at a position angle (P.A.) of 314 degrees +/- 8 degrees. At a distance of similar to 500 pc, the space velocity of the pulsar would be of similar to 390 km s(-1) assuming no inclination with respect to the plane of the sky. The direction of the pulsar proper motion is aligned very well with the main axis of the X-ray nebula (P.A. = 315 degrees.5 +/- 1 degrees.5), pointing to a physical, yet elusive, link between the nebula and the pulsar space velocity. No optical emission in the H alpha line is seen in a deep image collected at the Gemini telescope, which implies that the interstellar medium into which the pulsar is moving is fully ionized.