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Wide-field optical surveys have begun to uncover large samples of fast(t_rise < 5d), luminous (M_peak < -18), blue transients. While commonlyattributed to the breakout of a supernova shock into a dense wind, the greatdistances to the transients of this class found so far have hampered detailedinvestigation of their properties. We present photometry and spectroscopy froma comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the firstfast-luminous optical transient to be found in real time at low redshift. Ourfirst spectra (<2 days after discovery) are entirely featureless. A very broadabsorption feature suggestive of near-relativistic velocities develops between3-8 days, then disappears. Broad emission features of H and He develop after>10 days. The spectrum remains extremely hot throughout its evolution, and thephotospheric radius contracts with time (receding below R<10^14 cm after 1month). This behaviour does not match that of any known supernova, although arelativistic jet within a fallback supernova could explain some of the observedfeatures. Alternatively, the transient could originate from the disruption of astar by an intermediate-mass black hole, although this would requirelong-lasting emission of highly super-Eddington thermal radiation. In eithercase, AT2018cow suggests that the population of fast luminous transientsrepresents a new class of astrophysical event. Intensive follow-up of thisevent in its late phases, and of any future events found at comparabledistance, will be essential to better constrain their origins.

The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?