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arXiv · 2106.01556

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

Nikhil Sarin·Rachel Hamburg·Eric Burns·Gregory Ashton·Paul D. Lasky·Gavin P. Lamb
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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.

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Nikhil Sarin, Rachel Hamburg, Eric Burns, Gregory Ashton, Paul D. Lasky, Gavin P. Lamb. 2021-06-03. Low-efficiency long gamma-ray bursts: A case study with AT2020blt. https://doi.org/10.1093/mnras%2Fstac601

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