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

Abstract

AbstractWe present ZTF20aajnksq (AT 2020blt), a fast-fading (Δr = 2.4 mag in Δt = 1.3 d) red (g − r ≈ 0.6 mag) and luminous (M1626 = −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 tj = 1 d (observer-frame); (2) a luminous (LX = 1046 erg s−1) X-ray counterpart; and (3) luminous (Lν = 4 × 1031 erg s−1 Hz−1 at 10 GHz) radio emission. However, no GRB was detected in the 0.74 d 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 Eγ,iso < 7 × 1052 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 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 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. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys. This chapter is based on Ho, A. Y. Q., D. A. Perley, P. Beniamini, et al. (2020d). ZTF20aajnksq (AT 2020blt): A Fast Optical Transient at z ≈ 2.9 with No Detected Gamma-Ray Burst Counterpart. The Astrophysical Journal, 905(2), 98. https://doi.org/10.3847/1538-4357/abc34d. arXiv: 2006.10761 [astro-ph.HE].KeywordsGamma-ray burstsLong-duration gamma-ray burstsFast transientsFast optical transientsDirty fireballsOrphan afterglowsOff-axis gamma-ray burstsZwicky Transient FacilityRelativistic explosionsRelativistic stellar explosions