The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z = 0.27

We present ZTF18abvkwla (the "Koala"), a fast blue optical transient discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey. This event has a number of features in common with the groundbreaking transient AT2018cow: blue colors at peak (g−r ≈ −0.5), a short rise time from half-max of under two days, a decay time of only three days, a high optical luminosity (M_(g,peak) ≈ −20.8), a hot (≳ 40,000K) featureless spectrum at peak light, and a luminous radio counterpart (νL_ν ≳ 10⁴⁰ erg sec⁻¹ at 10 GHz). The radio luminosity of ZTF18abvkwla exceeds that of AT2018cow by an order of magnitude, and the late-time (Δt ≳ 80 days) light curve resembles that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy (M ≈ 5 × 10⁸ M_⊙, SFR ≈ 7 M_⊙ year⁻¹) that is moderately metal-enriched (log[O/H] ≈ 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 collimated ejecta and the optical from shock-interaction with confined dense material (< 0.07 M_⊙ in ∼10¹⁵ cm). Compiling transients in the literature with t_(rise) < 5 days and M_(peak) < −20, 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 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.