The Long, the Short and the Weak - the origin of GRBs

TL;DR

This paper investigates the origins of different types of Gamma-Ray Bursts, showing that low luminosity GRBs likely have a different origin than collapsar-based long GRBs, and revises duration-based classification criteria.

Contribution

It demonstrates that low luminosity GRBs cannot originate from collapsars and proposes revised duration thresholds for classifying GRBs based on detector type.

Findings

Low luminosity GRBs are not formed by collapsars.

Regular long GRBs are formed by collapsars.

Duration thresholds for GRB classification vary by detector.

Abstract

The origin of Gamma-Ray Bursts is one of the most interesting puzzles in recent astronomy. During the last decade a consensus formed that long GRBs (LGRBs) arise from the collapse of massive stars and that short GRBs (SGRBs) have a different origin, most likely neutron star mergers. A key ingredient of the Collapsar model that explains how the collapse of massive stars produces a GRB is the emergence of a relativistic jet that penetrates the stellar envelope. The condition that the emerging jet penetrates the envelope poses strong constraints on the system. Using these constraints we show that: (i) Low luminosity GRBs (llGRBs), a sub population of GRBs with a very low luminosities (and other peculiar properties: single peaked, smooth and soft) cannot be formed by Collapsars. llGRBs must have a different origin (most likely a shock breakout). (ii) On the other hand regular LGRBs must be formed by Collapsars. (iii) While for BATSE the dividing duration between Collapsars and non-Collapsar is indeed at $\sim 2$ sec, the dividing duration is different for other GRBs detectors. In particular most Swift bursts longer than 0.8 sec are of a Collapsar origin. This last results requires a revision of many conclusions concerning the origin of Swift SGRBs which were based on the commonly used 2 sec limit.