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

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arxiv 2003.01222 v4 pith:Z2W3BOJG submitted 2020-03-02 astro-ph.HE

classification astro-ph.HE
keywords opticalradiomathrmpeaktransientztf18abvkwlablueemission
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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 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.

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Forward citations

Cited by 8 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

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    astro-ph.HE 2025-08 conditional novelty 7.0 of 10

    SN 2023uqf, a rare interacting Type Ibn supernova found coincident with neutrino IC231004A, is the first observational evidence that interacting supernovae are hadronic accelerators.

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  4. Radio Follow-Up of Einstein Probe Fast X-Ray Transients

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    ATA and multi-facility radio follow-up of 20 EP FXTs yields two relativistic afterglows and mJy-level limits implying a heterogeneous FXT population overlapping bright GRB and TDE radio luminosities.

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    astro-ph.HE 2026-07 conditional novelty 6.0 of 10

    A wind-and-irradiation disk model fits the AT2018cow UV plateau with accretor masses from 1.4 to ~100 solar masses, removing the need for a >200 solar-mass black hole.

  6. Synchrotron Emission from Cooled Particle Distributions

    astro-ph.HE 2026-07 conditional novelty 6.0 of 10

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    astro-ph.HE 2026-08 accept novelty 5.0 of 10

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    astro-ph.HE 2026-07 conditional novelty 5.0 of 10

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