A new subclass of gamma-ray burst originating from compact binary merger

TL;DR

This paper identifies a new subclass of gamma-ray bursts, called type IL GRBs, originating from compact binary mergers with longer durations and distinct emission episodes, supported by analysis of recent and archival data.

Contribution

It introduces the concept of type IL GRBs, characterizes their unique three-episode emission pattern, and links them to the pre-merger precursor model, expanding understanding of merger-origin GRBs.

Findings

Type IL GRBs have a three-episode emission pattern.

GRB 170228A shares properties with GRB 211211A and GRB 230307A.

Pre-merger super flare model is favored for type IL GRBs.

Abstract

Type I gamma-ray bursts (GRBs) are believed to originate from compact binary merger usually with duration less than 2 seconds for the main emission. However, recent observations of GRB 211211A and GRB 230307A indicate that some merger-origin GRBs could last much longer. Since they show strikingly similar properties (indicating a common mechanism) which are different from the classic "long"-short burst (e.g. GRB 060614), forming an interesting subclass of type I GRBs, we suggest to name them as type IL GRBs. By identifying the first peak of GRB 230307A as a quasi-thermal precursor, we find that the prompt emission of type IL GRB is composed of three episodes: (1) a precursor followed by a short quiescent (or weak emission) period, (2) a long-duration main emission, and (3) an extended emission. With this burst pattern, a good candidate, GRB 170228A, was found in the Fermi/GBM archive data, and subsequent temporal and spectral analyses indeed show that GRB 170228A falls in the same cluster with GRB 211211A and GRB 230307A in many diagnostic figures. Thus this burst pattern could be a good reference for rapidly identifying type IL GRB and conducting low-latency follow-up observation. We estimated the occurrence rate and discussed the physical origins and implications for the three emission episodes of type IL GRBs. Our analysis suggests the pre-merger precursor model, especially the super flare model, is more favored for type IL GRBs.