Far-infrared and submillimetre observations of the Crab nebula

We present far-infrared (FIR) and submillimetre images of the Crab nebula, taken with the ISOPHOT instrument onboard the Infrared Space Observatory (ISO) and with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The ISOPHOT observations were made in the bands centred at 60, 100 and 170 µm, with FWHM of equivalent area Gaussian beams of 44, 47 and 93 arcsec, respectively. The 850-µm SCUBA image was processed using a maximum-entropy method algorithm and has a Gaussian FWHM of 17 arcsec. The 60- and 100-µm images show clear excess of emission, above that expected from an extrapolation of the synchrotron spectrum of the Crab nebula from lower frequencies, as previously seen from IRAS observations. The superior angular resolution of the ISOPHOT images reveal that about half this excess is attributable to two peaks, separated by ≈80 arcsec. We also present spectra taken using the Long-wavelength Spectrometer onboard ISO, which show that the FIR excess is not due to line emission. The lower-resolution 170-µm image does not show any excess emission, but is possibly fainter, particularly in the NW, than expected from an extrapolation of the lower-frequency synchrotron emission. These findings are consistent with a picture in which the FIR excess is due to emission from a small amount of ‐

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