Keck/LRIS optical spectrum of ZTF20aajnksq

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_=1d (observer frame); (2) a luminous (L_0.3-10KeV_=10^46^erg/s) X-ray counterpart; and (3) luminous (L_10GHz_=4x10^31^erg/s/Hz) radio emission.

However, no GRB was detected in the 0.74d between the last ZTF nondetection (r>21.36mag) and the first ZTF detection (r=19.60mag), with an upper limit on the isotropic-equivalent gamma-ray energy release of E_{gamma},iso_<7x10^52^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.