Low-efficiency long gamma-ray bursts: A case study with AT2020blt

TL;DR

This study analyzes AT2020blt, an optical transient likely caused by a long gamma-ray burst with unusually low gamma-ray efficiency, using detailed models and data from multiple observatories to understand its nature.

Contribution

It provides the first detailed modeling of AT2020blt as a long gamma-ray burst afterglow and establishes its low gamma-ray efficiency, expanding understanding of low-efficiency GRBs.

Findings

AT2020blt is best explained as an on-axis long gamma-ray burst afterglow.

The gamma-ray efficiency of AT2020blt is estimated to be less than 4.5%.

Low-efficiency long gamma-ray bursts are becoming more observable with new facilities.

Abstract

The Zwicky Transient Facility recently announced the detection of an optical transient AT2020blt at redshift $z=2.9$, consistent with the afterglow of an on-axis gamma-ray burst. However, no prompt emission was observed. We analyse AT2020blt with detailed models, showing the data are best explained as the afterglow of an on-axis long gamma-ray burst, ruling out other hypotheses such as a cocoon and a low-Lorentz factor jet. We search \textit{Fermi} data for prompt emission, setting deeper upper limits on the prompt emission than in the original detection paper. Together with KONUS-\textit{Wind} observations, we show that the gamma-ray efficiency of AT2020blt is $\lesssim 0.3 - 4.5\%$. We speculate that AT2020blt and AT2021any belong to the low-efficiency tail of long gamma-ray burst distributions that are beginning to be readily observed due to the capabilities of new observatories like the Zwicky Transient Facility.