AT 2018cow VLBI: No Long-Lived Relativistic Outflow

Abstract

We report on VLBI observations of the fast and blue optical transient (FBOT), AT 2018cow. At ~62 Mpc, AT 2018cow is the first relatively nearby FBOT. The nature of AT 2018cow is not clear, although various hypotheses from a tidal disruption event to different kinds of supernovae have been suggested. It had a very fast rise time (3.5 d) and an almost featureless blue spectrum although high photospheric velocities (40,000 km s${}^{-1}$) were suggested early on. The X-ray luminosity was very high, ~$1.4\times 10^{43}$ erg s${}^{-1}$, larger than those of ordinary SNe, and more consistent with those of SNe associated with gamma-ray bursts. Variable hard X-ray emission hints at a long-lived "central engine." It was also fairly radio luminous, with a peak 8.4-GHz spectral luminosity of ~$4\times 10^{28}$ erg s${}^{-1}$ Hz${}^{-1}$, allowing us to make VLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow. Assuming a circularly symmetric source, our observations constrain the average apparent expansion velocity to be <0.49$c$ by t = 98 d (3$\sigma$ limit). We also constrain the proper motion of AT 2018cow to be <0.51$c$. Since the radio emission generally traces the fastest ejecta, our observations make the presence of a long-lived relativistic jet with a lifetime of more than one month very unlikely.

Accepted for publication in MNRAS 3 figurs]es

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• Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV bright Underlying Source 2--4 Yr Post-discovery
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• Late-time Hubble Space Telescope Observations of AT 2018cow. I. Further Constraints on the Fading Prompt Emission and Thermal Properties 50--60 Days Post-discovery
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• Radio Supernovae
• AT2019ijn: a fast-rising, slow-decaying blue optical transient with exceptionally bright radio emission