Neutron Star-White Dwarf Merger as One Possible Optional Source of Kilonova-like Emission: Implications for GRB 211211A

TL;DR

This study investigates whether neutron star-white dwarf mergers can produce kilonova-like emissions similar to those observed in GRB 211211A, focusing on nucleosynthesis outcomes and emission modeling.

Contribution

It provides the first detailed nucleosynthesis simulations for NS-WD mergers and compares the resulting kilonova emissions with observations of GRB 211211A.

Findings

Heavy elements produced have mass number less than 90

NS-WD mergers can explain the kilonova if the remnant is a stable magnetar

Inability to produce heavy r-process elements in these mergers

Abstract

Long-duration GRB 211211A, which lacked an associated supernova at such a low redshift $z=0.076$, but was associated with a possible kilonova emission, has attracted great attention. The neutron star-white dwarf (NS-WD) merger is proposed as a possible progenitor of GRB 211211A, and it could naturally explain the long duration of the prompt emission. However, the NS-WD merger is not an ideal site for producing heavy elements via r-process nucleosynthesis. In this Letter, we investigate the heavy elements produced in NS-WD mergers based on numerical simulations of nucleosynthesis via SkyNet, and then calculate the resulting kilonova-like emission to compare with the solidly observed case of possible kilonova emission associated with GRB 211211A. By adopting three models (i.e., Model-A, Model-B, and Model-C) from \cite{2023ApJ...956...71K} at different temperatures ($T=4$ GK, 5 GK, and 6 GK), which are treated as free parameters, we find that the mass number of the heaviest element produced in our simulations is less than 90 ($A< 90$). Moreover, by comparing the calculated kilonova-like emission with the afterglow-subtracted observations of the possible kilonova associated with GRB 211211A, it is found that the merger of NS and WD cannot be ruled out as the origin of GRB 211211A to produce the possible kilonova emission if the remnant of the WD-NS merger is a supramassive or stable magnetar. Otherwise, it is difficult to explain the early possible kilonova emission following GRB 211211A by invoking the merger of a WD and an NS.