The thing appeared in May last year, while Muxlow and his colleagues were monitoring an unrelated stellar explosion in M82 using the MERLIN network of radio telescopes in the UK. A bright spot of radio emission emerged over only a few days, quite rapidly in astronomical terms. Since then it has done very little except baffle astrophysicists.
It certainly does not fit the pattern of radio emissions from supernovae: they usually get brighter over a few weeks and then fade away over months, with the spectrum of the radiation changing all the while. The new source has hardly changed in brightness over the course of a year, and its spectrum is steady.
His best guess is still that the radio source is some kind of dense object accreting surrounding material, perhaps a large black hole or a black hole in an unusual environment. Perhaps the phenomenon also happens occasionally in our galaxy, but is more common in M82 because it is a “starburst” galaxy – a cosmic cauldron where massive stars are forming and exploding at a much higher rate than in the Milky Way, creating a lot of new black holes.