Activity and radio spectral index variations near the center of the Crab nebula

A new high-resolution (1.8 arcsec) image of the 1.5-5 GHz spectral index of the Crab nebula, based on recent radio observations, and a 1.5-14 GHz spectral index image of the Crab with 6.5 arcsec resolution are presented. Small-scale, variable, wisp-like features are found near the center which exhibit a spectral index flatter (by about 0.05) than that of the rest of the nebula. They are part of an apparently organized complex of alpha perturbations which appear as tilted ring-like features having thickness no greater than 0.01 pc and diameter of order 0.6 pc. The more intense parts of these alpha features are also visible in total emission and seem to be associated with the highly variable features visible in the optical continuum (Scargle's wisps) although an exact positional coincidence is unlikely. They are seen to change both in intensity and position over the period 1982-1985. No changes in alpha are seen near the bright filaments, even up to a frequency of 14 GHz. In general, the spectral index does not vary greatly over the nebula.