Unraveling the Geometry of the Crab Nebula's "Inner Ring"
  Chandra images of the Crab Nebula resolve the detailed structure of its "inner ring", possibly a termination shock where pulsar-accelerated relativistic particles begin to emit X radiation. Analysis of these images finds that the center of the ellipse-presumably a circular ring in projection-lies about 0.9" (10 light-days at 2 kpc) from the pulsar's image, at a position angle of about 300{\deg} (East of North). This analysis also measures properties of the ellipse: The position angle of the semi-major axis is about 210{\deg} (East of North); the aspect ratio, 0.49.   In a simple-albeit, not unique-de-projection of the observed geometry, a circular ring is centered on the axis of symmetry of the pulsar wind nebula. This ring is not equatorial but rather lies near +4.5{\deg} latitude in pulsar-centered coordinates. Alternative geometries are briefly discussed. 
Martin C. Weisskopf, Ronald F. Elsner, Jeffery J. Kolodziejczak,   Stephen L. O'Dell and Allyn F. Tennant
astro-ph.HE