ZTF20aajnksq (AT 2020blt): A Fast Optical Transient at z ≈ 2.9 with No Detected Gamma-Ray Burst Counterpart
Anna Y. Q. Ho, Daniel A. Perley, Paz Beniamini, S. Bradley Cenko, S. R. Kulkarni, Igor Andreoni, Leo P. Singer, Kishalay De, Mansi M. Kasliwal, Christoffer Fremling, Eric C. Bellm, Richard Dekany, Alexandre Delacroix, Dmitry A. Duev, Daniel A. Goldstein, V. Zach Golkhou, Ariel Goobar, Matthew J. Graham, David Hale, Thomas Kupfer, Russ R. Laher, Frank J. Masci, Adam A. Miller, James D. Neill, Reed Riddle, Ben Rusholme, David L. Shupe, Roger Smith, Jesper Sollerman, Jan van Roestel
Abstract
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.