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

TL;DR

This work tests the WD–NS merger hypothesis for GRB 211211A by quantifying the intrinsic NS–WD vs NS–NS merger fraction within short GRB progenitors using a qualitative population-estimate framework based on massive-binary evolution. The authors derive a benchmark observational fraction $f_{ m NS-WD,obs} \approx 0.051$ and show that standard evolutionary paths yield $f_{ m NS-WD} \sim 14$–$37\%$ for fiducial WD masses, significantly higher than the observed rate. They explore refinements via WD mass constraints and alternative NS-formation channels (hypernova-born magnetars, accretion-induced collapse) that can bring model predictions in line with the ~5% fraction. The results imply either a sizable unidentified GRB 211211A–like population or rare NS formation channels, and they discuss implications for magnetar-driven central engines and broader compact-binary populations (NS–NS, NS–BH, BH–WD). The approach provides a reproducible, physics-informed lens to interpret rare GRB phenomena within the population-building framework of binary evolution, with potential to inform future gravitational-wave–gamma-ray coincidences.

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.

Figures (2)

• Figure 1: Schematic evolution of a massive binary towards to short $\gamma$-ray bursts (SGRBs; Path A) and GRB 211211A (Paths B and C). The meaning of the abbreviations are indicated in the following: 1) "MS", main-sequence, 2) "He star", helium star, 3) "WD", white dwarf, 4) "SN", supernova, 5) "NS", neutron star, 6) "CEE", common envelope evolution, 7) "Case BA/BB", mass transfer with a core helium burning/depleted mass donor, 8) "AIC", accretion-induced collapse, and 9) "SGRB", short gamma-ray burst.
• Figure 2: The calculated fractions of neutron star-white dwarf (NS-WD) binaries in NS-NS binaries as the function of the minimal zero-age main-sequence mass assumed for the WD. The corresponding WD masses are indicated on the top. The solid blue line, dashed blue line, and dotted blue line correspond to three evolutionary paths for the NS-WD binaries, which are the Standard case (Eq. \ref{['f_NSWD_pathB']}; Path B in Fig. \ref{['schematic']}), the hypernova case (Eq. \ref{['f_NSWD_hyper']}; Path B but at high-mass regime), and the accretion-induced collapse case (Eq. \ref{['f_NSWD_AIC']}; Path C in Fig. \ref{['schematic']}). In addition, the black vertical line indicates our fiducial value, and the horizontal orange line marks the 5% benchmark from observations, with the grey region indicating the confidence interval estimated by the Poisson error (Eq. \ref{['eq:fraction']}).