SN 1986J VLBI: The evolution and deceleration of the complex source and a search for a pulsar nebula

We report on VLBI observations of supernova 1986J in the spiral galaxy NGC 891 at two new epochs, 1990 July and 1999 February, t = 7.4 and 15.9 yr after the explosion, respectively, and on a comprehensive analysis of these and earlier observations from t similar to 4 yr after the explosion date, which we estimate to be 1983.2 +/- 1.1. The source is a shell or composite and continues to show a complex morphology with large brightness modulations along the ridge and with protrusions. The supernova is moderately to strongly decelerated. The average outer radius expands as t(0.71+/-0.11), and the expansion velocity has slowed to 6000 km s(-1) at t = 15.9 yr from an extrapolated 20,000 km s(-1) at t = 0.25 yr. The structure changes significantly with time, showing that the evolution is not self-similar. The shell structure is best visible at the latest epoch, when the protrusions have diminished somewhat in prominence and a new, compact component has appeared. The radio spectrum shows a clear inversion above 10 GHz. This might be related to a pulsar nebula becoming visible through the debris of the explosion. The radio flux density between 1.5 and 23 GHz decreases strongly with time, with the flux density proportional tot(-2.9+/-0.24) between t similar to 15 and 19 yr. This decrease is much more rapid than that found in earlier measurements up to t similar to 6 yr.