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The nearby luminous transient AT2018cow: a magnetar formed in a sub-relativistically expanding non-jetted explosion

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 year. With an astrometric precision up to 25 micro-arcseconds per epoch, the rapidly fading compact milli arcsecond scale source is found to be non-jetted with a proper motion of <= 0.15 mas/yr (0.14 c). This and a dense (number density ~ 10^4 - 10^5/cm^3) 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.