Multiwavelength Observations of the Extreme Transient AT2018cow

The bright, luminous, and fast multiwavelength transient AT2018cow displayed unprecedented behavior at almost every waveband: a fast-evolving UV/optical counterpart that maintains an extremely hot photospheric temperature over its entire evolution, an erratically-fluctuating luminous X-ray counterpart, and a heavily self-absorbed radio/submillimeter counterpart. We present a comprehensive set of photometric and spectroscopic observations of this transient from ground-based observatories spanning four continents and from space, including multiwavelength observations from Swift, NuSTAR, Astrosat, SMA, and ALMA. We consider both supernova and intermediate-mass black hole tidal disruption event interpretations for its origin, although no simple theoretical model can explain the full set of observations. Transients similar to AT2018cow are probably not rare in the universe and will be detected in large numbers by LSST in the coming decade.