Supernova 1986J: a Neutron Star or Black Hole in the Centre?

Supernova 1986J is almost the same age as SN 1987A, but was Type IIn, and likely had a massive progenitor. Located at 10 Mpc in NGC 891, it is one of the few supernovae whose radio emission can be resolved using VLBI. We present a new 5-GHz global-VLBI image of SN 1986J from 2014 as well as broadband VLA flux-density measurements. SN 1986J is unusual in that a compact synchrotron radio-emitting component appeared in the centre of the expanding shell of ejecta ~14 yr after the explosion, which now dominates the VLBI image. The central component is stationary to within the uncertainties (<570 km s^-1), and it has a marginally resolved HWHM radius of (6.7_-3.7 ^+0.7) × 10¹⁶ cm. The shell has expanded with average v ~= 5400 km s^-1. The central component's 5-GHz flux density is still increasing with time, and at present it has a 5-GHz νL _ν luminosity of ~4 × 10³⁵ erg s^-1, ~20 times that of the Crab Nebula. The central component may be due to a newly formed pulsar wind nebula, or an accreting black hole, or it may be due to interaction of the supernova shock with a highly structured environment left over from a progenitor which was in a close binary system. We discuss the newest observations and the constraints on its nature.