Magnetar Spin‐Down, Hyperenergetic Supernovae, and Gamma‐Ray Bursts

The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birthof a neutron star in a successful core-collapse supernova, is accompanied by aneutrino-driven wind. For magnetar-strength ($\sim10^{15}$ G) large scalesurface magnetic fields, this outflow is magnetically-dominated during theentire cooling epoch.Because the strong magnetic field forces the wind toco-rotate with the protoneutron star,this outflow can significantly effect theneutron star's early angular momentum evolution, as in analogous models ofstellar winds (e.g. Weber & Davis 1967). If the rotational energy is large incomparison with the supernova energy and the spindown timescale is short withrespect to the time required for the supernova shockwave to traverse thestellar progenitor, the energy extracted may modify the supernova shockdynamics significantly. This effect is capable of producing hyper-energeticsupernovae and, in some cases, provides conditions favorable for gamma raybursts. We estimate spindown timescales for magnetized, rotating protoneutronstars and construct steady-state models of neutrino-magnetocentrifugally drivenwinds. We find that if magnetars are born rapidly rotating, with initial spinperiods ($P$) of $\sim1$ millisecond, that of order $10^{51}-10^{52}$ erg ofrotational energy can be extracted in $\sim10$ seconds. If magnetars are bornslowly rotating ($P\gtrsim10$ ms) they can spin down to periods of $\sim1$second on the Kelvin-Helmholtz timescale.Comment: 16 pages, 5 figures, emulateap