GRB 200826A: A Precursor of a Long GRB with Missing Main Emission

TL;DR

This paper proposes that the short-duration GRB 200826A is actually a precursor to a longer gamma-ray burst, with the main emission missed due to geometric effects like jet precession or obstruction, explaining its peculiar characteristics.

Contribution

It introduces a model where missing main emission explains the short duration of certain long GRBs, supported by spectral and temporal similarities to known precursors.

Findings

GRB 200826A's properties resemble those of a known precursor in GRB 160625B.

The main emission could be hidden by jet precession or companion star obstruction.

This model explains the rarity and peculiar features of GRB 200826A-like bursts.

Abstract

The recently discovered peculiar gamma-ray burst GRB 200826A poses a dilemma for the collapsar model. Although all other characteristics of the burst are consistent with it being a Type II (i.e., collapse of a massive star) event, the observed duration of the event is only approximately 1 s, which is at odds with the predicted allowable timescale range for a collapsar event. To resolve this dilemma, this {\it Letter} proposes that the original burst could be an intrinsically long GRB comprising of a precursor and a main emission (ME) phase. However, the main emission phase is missed due to either precession of the jet or the obstruction of a companion star, leaving only the precursor observed as a short-duration GRB 200826A. Interestingly, we found that the temporal and spectral properties of GRB 200826A broadly resembled those of the bright precursor observed in GRB 160625B. Furthermore, assuming the prototype burst of GRB 200826A is similar to that of GRB 160625B, we found that the observer may indeed miss its main emission because of geometric effects caused either by jet precession or companion-obstruction models. Our approach provides a natural explanation for the GRB 200826A-like bursts and agrees with the rarity of those events.