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

We present ZTF20aajnksq (AT2020blt), a fast-fading (Δr = 2.4 mag in Δt = 1.3 days) red (g−r ≈ 0.6 mag) and luminous (M₁₆₂₆ = −25.9) optical transient at z = 2.9 discovered by the Zwicky Transient Facility (ZTF). AT2020blt 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 day (observer-frame); (2) a luminous (L_X = 10⁴⁶ erg s⁻¹) X-ray counterpart; and (3) luminous (L_ν = 4×10³¹ erg sec⁻¹ Hz−1 at 10 GHz) radio emission. However, no GRB was detected in the 0.74d 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_(γ,iso) < 7×10⁵² erg. AT2020blt 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 AT2020blt are consistent with a classical (initial Lorentz factor Γ₀ ≳ 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 AT2020blt 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.