GRB 211211A: A Neutron Star–White Dwarf Merger?

Abstract

The gamma-ray burst GRB 211211A and its associated kilonova-like emission were reported recently. A significant difference between this association event and GRB 170817A/AT 2017gfo is that GRB 211211A has a very long duration. In this Letter, we show that this association event may arise from a neutron star–white dwarf (NS–WD) merger if the central engine leaves a magnetar behind. Within the NS–WD merger, the main burst of GRB 211211A could be produced by magnetic bubble eruptions from toroidal magnetic field amplification of the premerger NS. This toroidal field amplification can be induced by the runaway accretion from the WD debris disk if the disk is in low initial entropy and efficient wind, while the extended emission of GRB 211211A is likely involved with magnetic propelling. The observed energetics and duration of the prompt emission of GRB 211211A can be fulfilled in comparison with those of accretion in a hydrodynamical thermonuclear simulation, as long as the WD has a mass ≳1M ⊙. Moreover, if the X-ray plateau in GRB afterglows is due to the magnetar spin-down radiation, GRB optical afterglows and kilonova-like emission can be jointly well modeled combining the standard forward shock with the radioactive decay power of 56Ni adding a rotational power input from the postmerger magnetar.

GRB 211211A-like Events and How Gravitational Waves May Tell Their Origins

“…With both ρ 0,GRB211211A and ρ NS−WD (beamed), one can estimate the fraction of NS-WD mergers that indeed make GRBs, i.e., With an N ∼ a few, one can see that f is a very small value, suggesting that only a small fraction of NS-WD systems are capable of producing GRBs (Yang et al 2022). This finding is in agreement with the calculation in Zhong et al (2023).…”

“…Yang et al (2022) pointed out that a neutron starwhite dwarf (NS-WD) merger involving a massive WD component near the Chandrasekhar limit and a similarly massive NS component is self-consistent with all the observations, ranging from prompt gamma-rays and early X-ray afterglow to the engine-fed kilonova emission. This scenario is supported by a more detailed modeling by Zhong et al (2023). We noticed that the scenario involving an NS-WD merger was not included in the analysis by Kunert et al (2023), which only compared the BNS merger, NS-BH merger, core-collapse supernova, and collapsar scenarios.…”

JWST detection of heavy neutron capture elements in a compact object merger

“…In particular, simulations show that no r-process material is produced [179], and so we should not expect the very red emission. Although there are suggestions that GRB 211211A could have been produced by a WD-NS merger [19,59], it is unclear if these could readily explain the detailed spectrophotometric evolution of GRB 230307A.…”

“…In particular, the natural timescales for emission in compact object mergers are much shorter than the measured duration of GRB 230307A. Previously suggested models that may also explain GRB 230307A include magnetars [55,56], black hole -neutron star mergers [57,58], or even neutron star -white dwarf systems [19,59]. Recent results have also shown that the jet timescale does not directly track the accretion timescale in compact object mergers, and that long GRBs may be created from very short lived engines [60], and hence from binary neutron star mergers without magnetars.…”

Evidence for Two Distinct Populations of Kilonova-associated Gamma-Ray Bursts

“…On the other hand, Zhu et al (2022) reported the fallback signatures in GRB 211211A, suggesting an NS-BH merger origin. Recent studies by Zhong et al (2023) claim that GRB 211211A could arise from an NS-WD merger if it produces a magnetar as remnant. They argue that the optical afterglows, along with KN-like emission, can be well modeled by combining the standard forward shock with the radioactive decay power of 56 Ni adding a rotational power input from the post-merger magnetar.…”