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

Abstract

The Kelvin-Helmholtz cooling epoch, lasting tens of seconds after the birth\nof a neutron star in a successful core-collapse supernova, is accompanied by a\nneutrino-driven wind. For magnetar-strength ($\\sim10^{15}$ G) large scale\nsurface magnetic fields, this outflow is magnetically-dominated during the\nentire cooling epoch.Because the strong magnetic field forces the wind to\nco-rotate with the protoneutron star,this outflow can significantly effect the\nneutron star's early angular momentum evolution, as in analogous models of\nstellar winds (e.g. Weber & Davis 1967). If the rotational energy is large in\ncomparison with the supernova energy and the spindown timescale is short with\nrespect to the time required for the supernova shockwave to traverse the\nstellar progenitor, the energy extracted may modify the supernova shock\ndynamics significantly. This effect is capable of producing hyper-energetic\nsupernovae and, in some cases, provides conditions favorable for gamma ray\nbursts. We estimate spindown timescales for magnetized, rotating protoneutron\nstars and construct steady-state models of neutrino-magnetocentrifugally driven\nwinds. We find that if magnetars are born rapidly rotating, with initial spin\nperiods ($P$) of $\\sim1$ millisecond, that of order $10^{51}-10^{52}$ erg of\nrotational energy can be extracted in $\\sim10$ seconds. If magnetars are born\nslowly rotating ($P\\gtrsim10$ ms) they can spin down to periods of $\\sim1$\nsecond on the Kelvin-Helmholtz timescale.\n