Linear growth of spiral SASI modes in core-collapse supernovae

Two-dimensional axisymmetric simulations have shown that the postbounce accretion shock in core-collapse supernovae is subject to the spherical accretion shock instability, or SASI. Recent three-dimensional simulations have revealed the existence of a nonaxisymmetric mode of the SASI as well, where the postshock flow displays a spiral pattern. Here we investigate the growth of these spiral modes using two-dimensional simulations of the postbounce accretion flow in the equatorial plane of a core-collapse supernova. By perturbing a steady state model we are able to excite both one-, two-, and three-armed spiral modes that grow exponentially with time, demonstrating that these are linearly unstable modes closely related to the original axisymmetric sloshing modes. By tracking the distribution of angular momentum, we show that these modes are able to efficiently separate the angular momentum of the accretion flow ( which maintains a net angular momentum of zero), leading to a significant spin-up of the underlying accreting proto-neutron star.