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, ~1.4×1043 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 ~4×1028 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 <0.49c by t = 98 d (3σ limit). We also constrain the proper motion of AT 2018cow to be <0.51c. 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.
@article{arxiv.1911.08778,
title = {AT 2018cow VLBI: No Long-Lived Relativistic Outflow},
author = {Michael F. Bietenholz and Raffaella Margutti and Deanne Coppejans and Kate D. Alexander and Megan Argo and Norbert Bartel and Tarraneh Eftekhari and Dan Milisavljevic and Giacomo Terreran and Edo Berger},
journal= {arXiv preprint arXiv:1911.08778},
year = {2020}
}