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 degrees (east of north). This analysis also measures properties of the ellipse: the position angle of the semi-major axis is about 210 degrees (east of north); the aspect ratio is 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.degrees 5 latitude in pulsar-centered coordinates. Alternative geometries are briefly discussed.