Optical Polarization and Intensity of the Pulsar in the Crab Nebula

Abstract

Broad-band photoelectric measurements on nine nights between 1969 February and 1970 February show constant intensity and colors for the pulsar of V = 16.5, B - V = +0.5, U - B = -0.45, and V - R = -0.75, with an accuracy of 10-15 percent in each passband. The ratio of the intensity in the main pulse to that in the subpulse is about 1.8 in each color, with the same accuracy. In the passband λλ3800-6000, the pulsar intensity repeated within 3 percent on nights 2 months apart. Measurements of linear polarization on four nights in 1969 February, August, and October show repeatable behavior, with the amount of polarization changing smoothly through the pulse from at least 25 percent to near zero and the plane of polarization sweeping through 150° during 60° of the pulsar's rotation. The time behavior was similar in the main pulse and the subpulse. The discontinuous change of the pulsar's period in 1969 September had no discernible effect upon the intensity, colors, or polarization of the pulses. It is argued that the observed pulses are due to a fixed, polarized emission pattern which is azimuthally scanned as the object rotates, and the sweep of the plane of polarization is interpreted in terms of a very general geometrical model. If the model is valid, then the projection of the rotation axis of the pulsar on the plane of the sky is found directly from the data, with an ambiguity of 90°, and is either parallel or orthogonal to the integrated magnetic field of the nebula within 20" of the pulsar. This agreement between the angles of the rotation axis and the magnetic field of the nebula seems too close to be accidental, and implies that the external field, at least in the vicinity of the pulsar, was produced by the pulsar at some stage in its history.

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