ZTF20aajnksq (AT2020blt): A Fast Optical Transient at $z\approx 2.9$ With No Detected Gamma-Ray Burst Counterpart

Abstract

We present ZTF20aajnksq (AT2020blt), a fast-fading ($\hat{I}”r=2.4$ mag in $\hat{I}”t=1.3$ days) 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). 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 $t_{\mathrm{j}}=1$ day (observer-frame); (2) a luminous $(L_X=10^{46}$ $\mathrm{erg}$ ${\mathrm{s}}^{-1})$ X-ray counterpart; and (3) luminous ($L_{\hat{I}}½=4\times 10^{31}$ $\mathrm{erg}$ ${\sec }^{-1}$ ${\mathrm{Hz}}^{-1}$ 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 $E_{{\hat{I}}^3,\mathrm{iso}}<7\times 10^{52}$ 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 $\hat{I}{“}_0\gtrsim 100$) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of 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.

19 pages, 4 figures, 1 table. Version accepted by ApJ on 17 Oct 2020