Diffusive acceleration of electrons in SN 1987A

We suggest that synchrotron radiation emitted by electrons undergoing diffusive acceleration at the supernova blast wave is the cause of the reappearance of radio emission from SN 1987A in 1990 July. Making reasonable assumptions concerning adiabatic losses and the magnetic field in the preexplosion stellar wind, we find the light curve and spectrum which would be produced by a constant rate of particle injection occurring in a localized region of the shock front; i.e., in a clump or knot. Observations indicate that two such clumps have been encountered to date. From the observed delay in the switch-on of the emission between 843 MHz and 4.8 GHz we find a spatial diffusion coefficient for electrons of κnormal_ = 2 x 10^20^ m^2^ s^-1^. This value agrees with that found for the second clump and is substantially greater than the Bohm value. Estimating the magnetic field to be 10^-7 T and assuming the diffusion coefficient is constant leads to the conclusion that the shock encountered the first clump on about day 900 and the second clump on day 1190. From the observed spectrum we find the compression ratio of the shock responsible for electron acceleration to be ~2.7. This implies strong modification of the shock front which may be due to the acceleration of cosmic rays.