The fast-rising blue optical transient AT2018cow indicated unusual early-phase characteristics unlike relatively better studied explosive transients. Its afterglow may be produced by either a relativistically beamed (jetted) or intrinsically luminous (non-jetted) ejecta and carries observational signatures of the progenitor and environment. High-resolution monitoring can distinguish between these scenarios and clarify the progenitor nature. We present very long baseline interferometry (VLBI) observations of AT2018cow at 5 GHz involving 21 radio telescopes from the European VLBI Network with five sessions spanning ≈1 yr. With an astrometric precision up to 25 μas per epoch, the rapidly fading compact mas-scale source is found to be non-jetted with a proper motion of ≤ 0.15 mas/yr (0.14 c). This and a dense magnetized environment (magnetic field strength ≥0.84 G) are characteristic of a newly formed magnetar driven central engine, originating in the successful explosion of a low-mass star.

Published in The Astrophysical Journal Letters, Volume 888, Issue 2, id.L24, 9 pp. (2020)