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

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 less than or similar to 0.3-4.5 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.

Automated summary

The Zwicky Transient Facility has detected an optical transient AT2020blt at redshift z = 2.9, which is consistent with the afterglow of an on-axis gamma-ray burst. Detailed analysis suggests that AT2020blt is best explained as the afterglow of an on-axis long gamma-ray burst, ruling out other hypotheses. The gamma-ray efficiency of AT2020blt is found to be less than or similar to 0.3-4.5 per cent.