Asymmetric supernovae and gamma-ray bursts

Abstract

Spectropolarimetry of core collapse supernovae has shown that they are asymmetric and often, but not universally, bi-polar. Jet-induced supernova models give a typical jet/torus structure that is reminiscent of some objects like the Crab nebula, SN 1987A and Cas A. Asymmetry in the strength of polar jets is a plausible mechanism to produce substantial pulsar “kick” velocities. Jets may arise from the intrinsic rotation and magnetic fields that are expected to accompany core collapse. We summarize the potential importance of the magneto-rotational instability (MRI) for the core collapse problem in the context of the non-monotonic behavior expected: increasing centrifugal support will lead to a maximum rotation and magnetic field production as a function of the initial rotation of the iron core. Non-axisymmetric instabilities are predicted for differentially rotating proto-neutron stars with values of the ratio of rotational kinetic energy to binding energy, T/∣ W∣≳0.01. The non-axisymmetric instabilities are likely to drive magnetosonic waves into the surrounding time-dependent density structure. These waves represent a mechanism of the dissipation of the rotational energy of the proto-neutron star, and the outward deposition of this energy may play a role in the supernova explosion process. The phase of deleptonization and contraction of the proto-neutron star lasting several seconds is likely to be an important phase of magnetic non-axisymmetric evolution. In the special circumstance that the proto-neutron star is born sufficiently rapidly rotating that it is subject to bar-mode instabilities on secular timescales, a possible outcome is that the deleptonizing neutron star will evolve along the locus T/∣ W∣∼0.14 releasing a significant fraction of its binding energy as MHD power sufficient to account for a GRB. This power will be provided over an extended time, 10 s, that is strongly reminiscent of the timescale of long GRBs and is also comparable to the time for a shock or jet to propagate through the core of a Type Ib/c supernova.