  The many unusual properties of the enigmatic AT2018cow suggested that at
least some subset of the empirical class of fast blue optical transients
(FBOTs) represents a genuinely new astrophysical phenomenon. Unfortunately, the
intrinsic rarity and fleeting nature of these events have made it difficult to
identify additional examples early enough to acquire the observations necessary
to constrain theoretical models. We present here the Zwicky Transient Facility
discovery of AT2020xnd (ZTF20acigmel, the "Camel") at z=0.243, the first
unambiguous AT2018cow analog to be found and confirmed in real time. AT2018cow
and AT2020xnd share all key observational properties: a fast optical rise,
sustained high photospheric temperature, absence of a second peak attributable
to ejection of a radioactively-heated stellar envelope, extremely luminous
radio, millimetre, and X-ray emission, and a dwarf-galaxy host. This supports
the argument that AT2018cow-like events represent a distinct phenomenon from
slower-evolving radio-quiet supernovae, likely requiring a different progenitor
or a different central engine. The sample properties of the four known members
of this class to date disfavour tidal disruption models but are consistent with
the alternative model of an accretion powered jet following the direct collapse
of a massive star to a black hole. Contextual filtering of alert streams
combined with rapid photometric verification using multi-band imaging provides
an efficient way to identify future members of this class, even at high
redshift.
