Magnetically driven explosions of rapidly rotating white dwarfs following accretion-induced collapse

We present two-dimensional multigroup flux-limited diffusion magnetohydrodynamics (MHD) simulations of the accretion-induced collapse (AIC) of a rapidly rotating white dwarf. We focus on determining the dynamical role of MHD processes after the formation of a millisecond-period proto-neutron star. We find that magnetic stresses lead to a powerful explosion with an energy of a few Bethe, rather than a weak one of at most 0.1 B, with an ejecta mass of ∼0.1 M_⊙, instead of a few 0.001 M_⊙. The core is spun down by ∼30% within 500 ms after bounce, and the rotational energy extracted is channeled into magnetic energy that generates a strong magneti-cally driven wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta, while this wind prevails, precludes it from becoming relativistic. This suggests that a γ-ray burst (GRB) is not expected to emerge from such AICs during the early proto-neutron star phase, except in the unlikely event that the massive white dwarf in this AIC context has sufficient mass to lead to black hole formation. In addition, we predict both negligible ⁵⁶Ni production (that should result in an optically dark, adiabatically cooled explosion), and the ejection of 0.1 M_⊙ of material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich nuclei puts strong constraints on the possible rate of such AICs. Moreover, being free from "fallback," such highly magnetized millisecond-period proto-neutron stars may, as they cool and contract, become magnetars, and the magnetically driven winds may later transition into Poynting-flux-dominated and relativistic winds, eventually detectable as GRBs at cosmological distances. The likely low event rate of AICs, however, will constrain them to be, at best, a small subset of GRB and/or magnetar progenitors.