English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

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. Thenature of AT 2018cow is not clear, although various hypotheses from a tidaldisruption event to different kinds of supernovae have been suggested. It had avery fast rise time (3.5 d) and an almost featureless blue spectrum althoughhigh photospheric velocities (40,000 km s$^{-1}$) were suggested early on. TheX-ray luminosity was very high, ~$1.4 \times 10^{43}$ erg s$^{-1}$, larger thanthose of ordinary SNe, and more consistent with those of SNe associated withgamma-ray bursts. Variable hard X-ray emission hints at a long-lived "centralengine." It was also fairly radio luminous, with a peak 8.4-GHz spectralluminosity of ~$4 \times 10^{28}$ erg s$^{-1}$ Hz$^{-1}$, allowing us to makeVLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow.Assuming a circularly symmetric source, our observations constrain the averageapparent expansion velocity to be <0.49$c$ by t = 98 d (3$\sigma$ limit). Wealso constrain the proper motion of AT 2018cow to be <0.51$c$. Since the radioemission generally traces the fastest ejecta, our observations make thepresence of a long-lived relativistic jet with a lifetime of more than onemonth very unlikely.

AT 2018cow VLBI: No Long-Lived Relativistic Outflow