A MODEL FOR THE RADIO-EMISSION FROM SNR 1987A

The observations of radio emission from supernova remnant SNR 1987A ca n be accounted for on the basis of diffusive shock acceleration of ele ctrons by the supernova blast wave. However, with this interpretation the observed spectral index implies that the compression ratio of the gas subshock is roughly 2.7 rather than the value of 4 expected of a s trong shock front. We propose that in SNR 1987A, ions also undergo dif fusive acceleration at the shock, a process that is likely to be rapid . Unlike the electron population, the accelerated ions can have an imp ortant effect on the gasdynamics. We calculate this coupled gas and en ergetic particle dynamics on the basis of the two-fluid model, in whic h the accelerated ions provide an additional component to the total pr essure acting on the fluid. By accelerating and possibly heating the u pstream plasma, the initially strong shock is modified and a weaker su bshock with an upstream precursor results. The electrons behave as tes t particles. They are accelerated at the evolving subshock, escape dow nstream, and emit synchrotron radiation in the swept up magnetic field . Two models are considered for the surroundings of the progenitor: th at of a freely expanding wind of number density n proportional to r(-2 ) and that of a wind confirmed by a shell of denser material, creating a stagnation zone of roughly constant density beyond the standing sho ck which terminates the free wind, We model the observed radio light c urves and the relatively steep spectrum of SNR 1987A using similar val ues for the ion acceleration parameters to those used in models of cos mic-ray acceleration in older SNRs, which can also contain high Mach n umber shocks, and find a good fit for the case in which the terminatio n shock is located at about 2 x 10(15) m from the progenitor.