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

Ho, AYQ, Perley, DA, Kulkarni, SR, Dong, DZJ, De, K, Chandra, P, Andreoni, I, Bellm, EC, Burdge, KB, Coughlin, M, Dekany, R, Feeney, M, Frederiks, DD, Fremling, C, Golkhou, VZ, Graham, MJ, Hale, D, Helou, G, Horesh, A, Kasliwal, MM , Laher, RR, Masci, FJ, Miller, AA, Porter, M, Ridnaia, A, Ben, R, Shupe, DL, Soumagnac, MT and Svinkin, DS (2020) The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z=0.27. Astrophysical Journal, 895 (1). ISSN 0004-637X

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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 ($g-r\approx -0.5$ 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 (gsim40,000 K) featureless spectrum at peak light, and a luminous radio counterpart. At late times (${\rm{\Delta }}t\gt 80\,\mathrm{days}$), the radio luminosity of ZTF18abvkwla ($\nu {L}_{\nu }\gtrsim {10}^{40}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ at 10 $\mathrm{GHz}$, observer-frame) is most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy ($M\approx 5\times {10}^{8}\,{M}_{\odot }$, $\mathrm{SFR}\approx 7\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$) that is moderately metal-enriched ($\mathrm{log}[{\rm{O}}/{\rm{H}}]\approx 8.5$), 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 (${\rm{\Gamma }}\beta c\gt 0.38c$) and the optical from shock-interaction with confined dense material (<0.07 M ⊙ in $\sim {10}^{15}\,\mathrm{cm}$). Compiling transients in the literature with ${t}_{\mathrm{rise}}\lt 5\,\mathrm{days}$ and ${M}_{\mathrm{peak}}\lt -20$ 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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical and Space Sciences, 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics, 0306 Physical Chemistry (incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society; IOP Publishing
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Date Deposited: 01 Jul 2020 10:01
Last Modified: 09 Sep 2020 11:45
DOI or Identification number: 10.3847/1538-4357/ab8bcf
URI: https://researchonline.ljmu.ac.uk/id/eprint/13218

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