GRB Progenitors and Observational Criteria

TL;DR

This paper reviews the classification of gamma-ray bursts (GRBs), discusses the challenges in identifying their progenitors, and proposes using amplitude as an additional criterion to better understand GRB origins and categories.

Contribution

It introduces the concept of amplitude as a new dimension for GRB classification and discusses the complexities in associating GRBs with their progenitors.

Findings

Long GRBs mostly originate from massive stars (Type II).

Short GRBs are generally linked to compact star mergers (Type I).

Amplitude can help distinguish GRB types, especially at high redshift.

Abstract

Phenomenologically, two classes of GRBs (long/soft vs. short/hard) are identified based on their gamma-ray properties. The boundary between the two classes is vague. Multi-wavelength observations lead to identification of two types of GRB progenitor: one related to massive stars (Type II), and another related to compact stars (Type I). Evidence suggests that the majority of long GRBs belong to Type II, while at least the majority of nearby short GRBs belong to Type I. Nonetheless, counter examples do exist. Both long-duration Type I and short-duration Type II GRBs have been observed. In this talk, I review the complications in GRB classification and efforts in diagnosing GRB progenitor based on multiple observational criteria. In particular, I raise the caution to readily accept that all short/hard GRBs detected by BATSE are due to compact star mergers. Finally, I propose to introduce "amplitude" as the third dimension (besides "duration" and "hardness") to quantify burst properties, and point out that the "tip-of-iceberg" effect may introduce confusion in defining the physical category of GRBs, especially for low-amplitude, high-redshift GRBs.