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

The second brightest gamma-ray burst, GRB 230307A (with a duration T 90 similar to 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 similar to 10-3 M circle dot 56Ni is involved in the ejecta of a mass of similar to 0.1 M circle dot. Particularly, the magnetar parameters required by the fit are consistent with those derived from the early X-ray observation.