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.

中文翻译：

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考拉：来自 z = 0.27 的星暴矮星系的具有发光无线电发射的快速蓝色光学瞬变

我们展示了 ZTF18abvkwla（“考拉”），这是一种在 Zwicky 瞬态设施 (ZTF) 一日节奏 (1DC) 调查中发现的快速蓝色光学瞬变。ZTF18abvkwla 与突破性的瞬态 AT2018cow 具有许多共同特征：峰值时的蓝色（$gr\approx-0.5$ mag），从半峰值的上升时间不到两天，衰减时间到半峰值的时间为仅三天，高光度（$M_{g,\mathrm{peak}}\approx-20.6$mag），峰值光下的热（$\gtrsim 40,000$K）无特征光谱，以及发光的无线电对应物。在后期（$\Delta t>80$d），ZTF18abvkwla 的射电光度（$\nu L_\nu \gtrsim 10^{40}$erg/s at 10 GHz，观察者坐标系）与长期伽马射线暴 (GRB)。宿主星系是一个矮星暴星系 ($M\approx5\times10^{8}M_\odot$, $\mathrm{SFR}\approx7 M_\odot$/yr) 中度金属富集（$\log\mathrm{[O/H]} \approx 8.5$），类似于 GRB 和超发光超新星的宿主。与 AT2018cow 一样，ZTF18abvkwla 中的无线电和光发射可能来自两个独立的部分：来自快速移动的喷射物的无线电（$\Gamma \beta c > 0.38c$）和来自与密闭致密材料的冲击相互作用的光学（$ <0.07M_\odot$ in $\sim 10^{15}$cm)。用 $t_\mathrm{rise} <5$d 和 $M_\mathrm{peak}<-20$mag 编译文献中的瞬态，我们发现很大一部分是引擎驱动的，并表明高峰值光学光度直接关系到这个引擎的存在。从 1DC 调查的 18 个月起，我们发现在这个上升-亮度相空间中的瞬变至少比 CC SNe 少两到三个数量级。最后，我们讨论了使用大型天气巡天望远镜等未来设施识别此类事件的策略，以及探测伴随 X 射线和无线电发射的前景。