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

Abstract

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 AT 2018cow: blue colors at peak (g – r ≈ −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,peak) ≈ −20.6 mag), a hot (≳40,000 K) featureless spectrum at peak light, and a luminous radio counterpart. At late times (Δt > 80 days), the radio luminosity of ZTF18abvkwla (ν_(Lν) ≳ 10⁴⁰ erg⁻¹ 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 ≈ 5×10⁸ M⊙, SFR ≈ 7 M⊙ yr⁻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 ejecta (Γβc > 0.38c) 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 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, as well as prospects for detecting accompanying X-ray and radio emission.

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