What Powered the Kilonova-like Emission after GRB 230307A in the Framework of a Neutron Star–White Dwarf Merger?

Abstract

The second brightest gamma-ray burst, GRB 230307A (with a duration T 90 ∼ 40 s), exhibited characteristics indicative of a magnetar engine during the prompt emission phase. Notably, a suspected kilonova was identified in its follow-up optical and infrared observations. Here we propose that the origin of GRB 230307A is a neutron star–white dwarf (NS–WD) merger as this could naturally explain the long duration and the large physical offset from the center of its host galaxy. In the framework of such an NS–WD merger event, the late-time kilonova-like emission is very likely to be powered by the spin-down of the magnetar and the radioactive decay of 56Ni, rather than by the decay of r-process elements as these heavy elements may not easily be synthesized in an NS-WD merger. It is demonstrated that the above scenario can be supported by our fit to the late-time observational data, where a mass of ∼10−3 M ⊙ 56Ni is involved in the ejecta of a mass of ∼0.1 M ⊙. Particularly, the magnetar parameters required by the fit are consistent with those derived from the early X-ray observation.