SN 1986J VLBI: II. The Evolution of the Shell and the Central Source
  We present new VLBI images of supernova 1986J, taken at 5, 8.4 and 22 GHz
between t=22 to 25 yr after the explosion. The shell expands as t^(0.69+-0.03).
We estimate the progenitor's mass-loss rate at (4 ~ 10) * 10^-5 Msol/yr (for
v_w = 10 km/s). Two bright spots are seen in the images. The first, in the
northeast, is now fading. The second, very near the center of the projected
shell and unique to SN1986J, is still brightening relative to the shell, and
now dominates the VLBI images. It is marginally resolved at 22 GHz (diameter
~0.3 mas; ~5 * 10^16 cm at 10 Mpc). The integrated VLA spectrum of SN1986J
shows an inversion point and a high-frequency turnover, both progressing
downward in frequency and due to the central bright spot. The optically-thin
spectral index of the central bright spot is indistinguishable from that of the
shell. The small proper motion of 1500+-1500 km/s of the central bright spot is
consistent with our previous interpretation of it as being associated with the
expected black-hole or neutron-star remnant. Now, an alternate scenario seems
also plausible, where the central bright spot, like the northeast one, results
when the shock front impacts on a condensation within the circumstellar medium
(CSM). The condensation would have to be so dense as to be opaque at cm
wavelengths (~1000x denser than the average corresponding CSM) and fortuitously
close to the center of the projected shell. We include a movie of the evolution
of SN1986J at 5 GHz from t=0 to 25 yr.
