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

TL;DR

This study uses VLBI observations to show that AT2018cow was powered by a newly formed magnetar in a dense environment, supporting a non-jetted explosion origin from a low-mass star.

Contribution

It provides high-precision astrometric evidence that AT2018cow was a non-jetted explosion driven by a magnetar, clarifying its progenitor and explosion mechanism.

Findings

AT2018cow is non-jetted with proper motion <= 0.15 mas/yr.

The environment has a high density (~10^4 - 10^5/cm^3) and strong magnetic fields (>= 0.84 G).

The explosion originated from a low-mass star forming a magnetar.

Abstract

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.