  We present ZTF18abvkwla (the "Koala"), a fast blue optical transient
discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey.
ZTF18abvkwla has a number of features in common with the groundbreaking
transient AT2018cow: blue colors at peak ($g-r\approx-0.5$ mag), a short rise
time from half-max of under two days, a decay time to half-max of only three
days, a high optical luminosity ($M_{g,\mathrm{peak}}\approx-20.6$mag), a hot
($\gtrsim 40,000$K) featureless spectrum at peak light, and a luminous radio
counterpart. At late times ($\Delta t>80$d) the radio luminosity of
ZTF18abvkwla ($\nu L_\nu \gtrsim 10^{40}$erg/s at 10 GHz, observer-frame) is
most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy
is a dwarf starburst galaxy ($M\approx5\times10^{8}M_\odot$,
$\mathrm{SFR}\approx7 M_\odot$/yr) that is moderately metal-enriched
($\log\mathrm{[O/H]} \approx 8.5$), similar to the hosts of GRBs and
superluminous supernovae. As in AT2018cow, the radio and optical emission in
ZTF18abvkwla likely arise from two separate components: the radio from
fast-moving ejecta ($\Gamma \beta c >0.38c$) and the optical from
shock-interaction with confined dense material ($<0.07M_\odot$ in $\sim
10^{15}$cm). Compiling transients in the literature with $t_\mathrm{rise} <5$d
and $M_\mathrm{peak}<-20$mag, we find that a significant number are
engine-powered, and suggest that the high peak optical luminosity is directly
related to the presence of this engine. From 18 months of the 1DC survey, we
find that transients in this rise-luminosity phase space are at least two to
three orders of magnitude less common than CC SNe. Finally, we discuss
strategies for identifying such events with future facilities like the Large
Synoptic Survey Telescope, and prospects for detecting accompanying X-ray and
radio emission.
