AT 2018cow VLBI: No long-lived relativistic outflow

Michael F. Bietenholz*, Raffaella Margutti, Deanne Coppejans, Kate D. Alexander, Megan Argo, Norbert Bartel, Tarraneh Eftekhari, Dan Milisavljevic, Giacomo Terreran, Edo Berger

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

Abstract

We report on Very Long Baseline Interferometry (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 supernovae (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 <0.49, c by t = 98 d (3σ 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 1 month very unlikely.

Original languageEnglish (US)
Pages (from-to)4735-4741
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Volume491
Issue number4
DOIs
StatePublished - Feb 1 2020

Keywords

  • radio continuum: general
  • supernovae: individual: AT 2018cow
  • transients: supernovae

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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