We present results of observations of the Crab Nebula and G21.5–0.9, performed at 1.3 mm with the MPIfR bolometers arrays at the IRAM 30 m telescope. In the Crab Nebula we measure spatial variations of the average spectral index between 20 cm and 1.3 mm. Since the electrons emitting at mm wavelengths are affected by negligible synchrotron losses, such variations imply the presence of at least two different populations of injected particles. By subtracting the emission extrapolated from the radio a residual component appears, similar in size and shape to the soft X-ray map as well as to the flatter-spectrum optical component (Véron-Cetty and Woltjer 1993). Moreover near the major synchrotron filaments we measure a spectral bending consistent with a break at a frequency lower than the average break frequency in the Nebula: this indicates that near the filaments the magnetic field is typically 6 times higher than the average. For G21.5–0.9 we derive a spectral break at ∼540 GHz, in contrast with the previously accepted value of 40 GHz. Therefore this object does not strictly belong to the class of “plerions with a low-frequency break.”

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