Low-efficiency long gamma-ray bursts: a case study with AT2020blt. Issue 1 (8th March 2022)

• Record Type: Journal Article
• Authors: Sarin, N; Hamburg, R; Burns, E; Ashton, G; Lasky, P D; Lamb, G P
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 512:Issue 1(2022)
• Page Start: 1391
• Page End: 1399
• Publication Date: 2022-03-08
• Subjects: gamma-ray bursts; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac601
• Languages: English
• ISSNs: 0035-8711

Abstract

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 Fermi data for prompt emission, setting deeper upper limits on the prompt emission than in the original detection paper. Together with KONUS- Wind observations, we show that the gamma-ray efficiency of AT2020blt is $\lesssim 0.3\!-\!4.5{{\ \rm per\, cent}}$ . 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.