THE ACCELERATION OF ELECTRONS IN THE RADIO SUPERNOVA SN 1986J

We propose a model for radio supernovae (RSN) based on the synchrotron emission from relativistic electrons which are diffusively accelerate d at the expanding supernova shock. This model was originally develope d for application to the optically thin emission observed from SN1987A . Here we generalise it by including the effects of free-free absorpti on from both an external screen and from material internal to the sour ce, and by relaxing the restriction to an azimuthal B-field. We find a good fit to the entire set of radio data for the best observed highly -luminous RSN - SN1986J - with a reduced chi(2) of 3.85. Applying the new model to SN1988Z, another intrinsically bright RSN, also yields a good fit (chi(red)(2) approximate to 2) but this is less significant, because of the limited data on this distant (z = 0.02) source. These f its suggest that the shock expands at constant speed, that the magneti c field within the source decreases with time according to t(-2), and that the compression ratio of the shock front is close to the value ex pected of a strong shock in an ideal gas of adiabatic index 5/3 - indi cating a relatively low value of the cosmic ray pressure compared with SN1987A. In the case of SN1986J we derive an explosion date in August /September 1982, a magnetic field at the position of the shock 1000 da ys after explosion of B approximate to 4 nT and a spatial diffusion co efficient of the electrons of kappa approximate to 4 x 10(19) m(2) s(- 1) four orders of magnitude greater than the Bohm value. In addition, me obtain the optical depths to external and internal absorption, and derive an estimate of the mass-loss rate.