A VLBI map at 1661 MHz of the brightest radio source in M82

A map of 41.9 + 58, the brightest radio source in M82, made with data from the European VLBI network at 1.66 GHz, shows an elongated structure of length (FWHM) of the order of 20 milliarcsec (of the order of 0.30 pc) in pa 47deg. Transatlantic VLBI data at the same frequency reveal that there is a more compact core (of the order of 4 milliarcsec) embedded within the source, with the extended structure rather symmetrically disposed on either side of it. 5-GHz data yield roughly the same estimate of overall size and position angle as at 1.66 GHz, showing that the effects of interstellar scattering on the more extended component are negligible. It is found that the decay of flux density of the source at 1.66 GHz must be mainly due to the extended component, and that variations may be more erratic than suggested by previous investigators. (author)