High-resolution polarization images of Crab Nebula with a charge-coupled device camera

Abstract

Optical linear polarization of the Crab Nebula supernova remnant is presumed to be a good example of the astrophysical occurrence of synchrotron emission from relativistic electrons in a magnetic field^1–3. However, there have been few attempts to map the spatial distribution of the polarization at the highest possible resolution and with high signal-to-noise ratio, although such observations contain information about the geometry and the small-scale structure of the magnetic field. We describe here observations of the Crab Nebula using a new charge coupled device (CCD) imaging/spectropolarimeter. Two large regions each 150 arc s across were observed with a seeing-limited spatial resolution of 1.5 arc s: the ‘eastern bay’ and the central wisps. The pulsar was included in both fields. In general, we find no evidence for substantial small-scale (1.5 arc s) structure in the projected magnetic field, but there are changes on scales of the order of 5 arc s which would be barely discernible by photographic methods. In addition, there is remarkable organization of the field over domains ranging in size from 10 to 30 arc s. The uncertainty about, the nebular polarization component near the pulsar is resolved. The presence or absence of small-scale structure in the field is important in understanding the energy transport mechanism for the relativistic electrons, and little is known about the connection between small polarization features and the remarkable filamentary structure in the nebula. Photographic maps² had a resolution of only 5.25 arc s and rather limited polarimetric accuracy; more recent photoelectric measurements⁴ yielded 1 × 3 arc s resolution but only in a small patch. Instrumental technique is crucial.