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 AT 2018cow : blue colors at peak ( g - r \approx - 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, \mathrm { peak } } \approx - 20.8 ) , a hot ( \gtrsim 40 , 000 K ) featureless spectrum at peak light , and a luminous radio counterpart ( \nu L _ { \nu } \gtrsim 10 ^ { 40 } \mbox { $ erg$ } \mbox { $ s ^ { -1 } $ } at 10 GHz ) .
The radio luminosity of ZTF18abvkwla exceeds that of AT2018cow by an order of magnitude , and the late-time ( \Delta t \gtrsim 80 \mbox { $ d$ } ) light curve resembles that of long-duration gamma-ray bursts ( GRBs ) .
The host galaxy is a dwarf starburst galaxy ( M \approx 5 \times 10 ^ { 8 } \mbox { $M _ { \odot } $ } , \mathrm { SFR } \approx 7 \mbox { $M _ { \odot } $ } \mbox { $ yr ^ { -1 } $ } ) 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 collimated ejecta and the optical from shock-interaction with confined dense material ( < 0.07 M _ { \odot } in \sim 10 ^ { 15 } \mbox { $ cm$ } ) .
Compiling transients in the literature with \mbox { $ t _ { rise } $ } < 5 \mbox { $ d$ } and M _ { \mathrm { 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 .