Tracing the Physical Lineage of GRB 211211A: Population Constraints on NS-WD Merger Gamma-Ray Bursts

TL;DR

This paper investigates the origin of the peculiar long gamma-ray burst GRB 211211A, proposing it results from a white dwarf-neutron star merger, and analyzes the expected versus observed merger rates to understand its rarity and implications.

Contribution

It provides a qualitative analysis of binary evolution physics to estimate the NS-WD/NS-NS merger fraction and compares it with observed rates, suggesting possible unseen populations or alternative origins.

Findings

Estimated NS-WD/NS-NS merger fraction is 14-37%, higher than the observed 5%.

Discrepancy suggests a large, unidentified population of similar GRBs.

Implications for black-hole binary systems are also discussed.

Abstract

The peculiar long gamma-ray burst (GRB) event, GRB 211211A, is known for it is association with a kilonova feature. Whereas most long GRBs are thought to originate in the core collapse of massive stars, the presence of kilonova suggests GRB 211211A was instead produced by a merger of a compact object binary. Building on the interpretation put forward by \citet{Yang2022Natur.612..232Y}--who argue that GRB 211211A was powered by a massive white-dwarf + neutron-star (WD-NS) merger--we adopt this WD-NS scenario as our observationally supported starting point. If the burst truly originates from that channel, its rarity must mirror the formation and merger rate of WD-NS binaries--a rate still largely unexplored in conventional massive-binary population studies. In this letter, we present a qualitative analysis based on binary evolution physics in order to understand the fraction of GRB 211211A in short GRBs (NS-WD/NS-NS fraction). Since the progenitors of massive WD-NS binaries occupy the initial mass function-preferred regime, where the zero-age main-sequence mass range of the assumed WD mass range (1.2-1.4$\,M_\odot$) is comparable to that of NSs, the NS-WD/NS-NS fraction emerging from our standard evolutionary path is expected to be $\sim$14--37\%, far higher than the observed fraction ($\sim5$\%). This discrepancy might imply a large, still-unidentified population of GRB 211211A-like events or an unusual origin of the NS-such as being hypernova-born or accretion-induced-collapse-born. Placing these results in a broader compact-binary context, implications for black-hole systems are also discussed.