Yves A. Gallant 1 & Richard J. Tuffs 2
1 Astronomical Institute, Utrecht University
2 Max-Plank-Institut fur Kernphysik, Heidelberg
We observed a number of plerionic (filled-center) supernova remnants with ISOCAM and ISOPHOT. In contrast to shell-type SNRs, these objects, of which the Crab Nebula is the best studied example, exhibit a filled- center morphology at radio and X-ray wavelengths, where the emission is thought to be synchrotron radiation from relativistic electrons and positrons accelerated by a central pulsar. Apart from the Crab Nebula itself, however, these objects had generally not been observed outside the radio and X-ray wavebands, despite the belief that the synchrotron spectra must be continuous between these frequency domains. Infrared wavelengths are more favourable than optical for detection of extended synchrotron emission because of the relative absence of interstellar extinction and the lesser extent of stellar confusion. Motivated by these considerations we selected several bright, compact plerions for observation with ISO, based on a simple extrapolation of their radio and X-ray power-law spectra.
Of three plerionic SNRs that were observed by ISO, two show clear evidence of extended emission when observed with ISOCAM in several broad-band filters, and on the basis of its morphology and spectral characteristics we interpret this emission as synchrotron radiation. ISOCAM observations of these fields were performed in three filters between 4 and 16 microns, using 6 arcsecond pixels but in each case performing a 3x3 micro-scan with fractional pixel oversampling, to yield an effective sampling of 2 arcseconds in the fully oversampled region of each image. The good sampling of the point-spread function resulting from this method allowed us to effectively fit and remove the field stars that add to the extended emission, especially at the shorter wavelengths. Comparison with the fluxes from these stars in ground-based images at 2 microns then allows an independent check on the absolute calibration of the ISOCAM intensities.
In the case of G21.5-0.9, a Galactic plerion, the infrared fluxes are consistent with a simple extrapolation from the radio and X-ray power-law synchrotron spectra. The infrared fluxes of the plerionic component the `Crab twin' 0540-69.3 in the Large Magellanic Cloud, however, suggest a third power-law component with spectral index intermediate between the radio and X-ray indices, as is observed in the Crab Nebula itself. Upper limits on non-stellar emission from the plerionic component of the third observed SNR, G29.7-0.3, also suggest an inflection between the radio and X-ray components of its spectrum. We briefly discuss the theoretical implications of these observations for the history of the synchrotron-emitting particles accelerated in these remnants. The last two of the above SNRs are of composite type, meaning that they comprise a shell-type component in addition to the plerionic ones discussed above. In both cases, there is evidence in the ISOCAM images for faint emission from the shell component as well, and we briefly discuss its possible origin.