We present ZTF20aajnksq ( AT 2020blt ) , a fast-fading ( \Delta r = 2.4 mag in \Delta t = 1.3 d ) red ( g - r \approx 0.6 mag ) and luminous ( M _ { 1626 } = -25.9 ) optical transient at z = 2.9 discovered by the Zwicky Transient Facility ( ZTF ) .
AT 2020blt shares several features in common with afterglows to long-duration gamma-ray bursts ( GRBs ) : ( 1 ) an optical light curve well-described by a broken power-law with a break at t _ { \mathrm { j } } = 1 d ( observer-frame ) ; ( 2 ) a luminous ( L _ { X } = 10 ^ { 46 } \mathrm { erg } \mathrm { s } ^ { -1 } ) X-ray counterpart ; and ( 3 ) luminous ( L _ { \nu } = 4 \times 10 ^ { 31 } \mbox { $ erg$ } \mbox { $ s ^ { -1 } $ } \mbox { $ Hz ^ % { -1 } $ } at 10 GHz ) radio emission .
However , no GRB was detected in the 0.74 d between the last ZTF non-detection ( r > 20.64 ) and the first ZTF detection ( r = 19.57 ) , with an upper limit on the isotropic-equivalent gamma-ray energy release of E _ { \gamma, \mathrm { iso } } < 7 \times 10 ^ { 52 } erg .
AT 2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart ( after PTF11agg and ZTF19abvizsw ) and the second ( after ZTF19abvizsw ) with a redshift measurement .
We conclude that the properties of AT 2020blt are consistent with a classical ( initial Lorentz factor \Gamma _ { 0 } \gtrsim 100 ) on-axis GRB that was missed by high-energy satellites .
Furthermore , by estimating the rate of transients with light curves similar to that of AT 2020blt in ZTF high-cadence data , we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs .
We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys .