The Radio Spectral Index of the Crab Nebula

Abstract

We present the results of a new, comprehensive investigation of the radio spectral index of the Crab Nebula supernova remnant. New data at 74 MHz are combined with data at 327 MHz, 1.5 GHz and 5 GHz. In contrast to previous claims, little spatial variation in the spectral index is seen. In particular, between 327 MHz and 5 GHz we see no evidence of spectral steepening near the edge of the nebula, the ``jet'' or the ionized filaments. The rms limits on any spectral index variations in these regions amount to no more than 0.01. We believe that earlier reports of large steepening were the result of correlator bias and image registration problems. An elongated feature was detected 1\arcmin northwest of the pulsar which may be a continuation of the well-known wisp-like structures seen closer to the center of the nebula. At 74 MHz, we see for the first time evidence of free-free absorption by the thermal material in the Crab Nebula's filaments. Apart from some possible renewed acceleration occurring in the wisps, the dominant accelerator of relativistic electrons in the Crab Nebula is the pulsar itself.