Far-infrared and sub-mm observations of the Crab nebula
  We present FIR and sub-mm images of the Crab nebula, taken with ISOPHOT on board ISO and with the SCUBA on the JCMT. The ISOPHOT observations were made at 60, 100, 170 micron, with FWHM of equivalent area Gaussian beams of 44, 47 and 93 arcsec, respectively. The 850 micron SCUBA image has a Gaussian FWHM of 17 arcsec. The 60 and 100 micron images show clear excess of emission above that expected from an extrapolation of the synchrotron spectrum of the Crab nebula from lower frequencies. The ISOPHOT images reveal that about half this excess is attributable to two peaks, separated by approximately 80 arcsec. We also present spectra taken using the LWS on board ISO, which show that the FIR excess is not due to line emission. The lower resolution 170 micron image does not show any excess emission. These findings are consistent with a picture in which the FIR excess is due to emission from a small amount of warm dust which radiates predominantly at 60 and 100 micron micron but not at 170 micron, and which is superimposed on a synchrotron spectrum which gradually steepens towards shorter wavelengths throughout the FIR and MIR spectral range. The dust geometry is consistent with a torus of diameter approximately 0.8 pc created by the supernova progenitor, superimposed upon a broadly distributed component which may be supernova condensates in the filaments. The dust-to-gas ratio in the filaments is comparable to the interstellar value. To study spectral index variations in the radio synchrotron emission we compared the 850-micron image with a 20-cm VLA image. These images are very similar, implying that there is little variation in spectral index across the face of the remnant between these wavelengths. 
D. A. Green and R. J. Tuffs and C. C. Popescu