ZTF20aajnksq (AT 2020blt): A Fast Optical Transient at z ≈ 2.9 with No Detected Gamma-Ray Burst Counterpart

Abstract

We present ZTF20aajnksq (AT 2020blt), a fast-fading (Δ r  = 2.3 mag in Δ t  = 1.3 days) red ( g  −  r  ≈ 0.6 mag) and luminous ( M 1626 Å  = −25.9 mag) 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 j  = 1 d (observer frame); (2) a luminous ( L 0.3–10 KeV  = 10 46 erg s −1 ) X-ray counterpart; and (3) luminous ( L 10 GHz  = 4 × 10 31 erg s −1 Hz −1 ) radio emission. However, no GRB was detected in the 0.74 days between the last ZTF nondetection ( r  > 21.36 mag) and the first ZTF detection ( r  = 19.60 mag), with an upper limit on the isotropic-equivalent gamma-ray energy release of E γ ,iso  < 7 × 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 Γ 0  ≳ 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, such as dirty fireballs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys.