MG11 
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General relativistic simulations of collapse and black hole formation in magnetized, differentially rotating neutron stars

Many problems at the forefront of theoretical astrophysics involve magnetized fluids in dynamical spacetimes. To address such problems, we have developed a code which evolves the full Einstein-Maxwell-MHD system of equations. We have subjected this code to a suite of test problems, including the excitation of MHD waves by linearized gravitational waves. Using this code, we explore the evolution of differentially rotating neutron stars with initially small magnetic fields. Of particular significance is the behavior found for hypermassive neutron stars (HMNSs), which have rest masses larger than the limit for uniform rotation. We find that secular angular momentum transport due to MHD effects results in collapse to rotating black holes surrounded by massive accretion tori. To survey the range of alternative outcomes, we also evolve magnetized neutron star models with lower masses and angular momenta. Instead of collapsing, these non-hypermassive models approach uniform rotation and, in cases with significant angular momentum, are surrounded by massive tori.

Magnetized hypermassive neutron star collapse: a candidate central engine for short-hard GRBs

Hypermassive neutron stars (HMNSs) are equilibrium configurations supported against collapse by rapid differential rotation and likely form as transient remnants of binary neutron star mergers. Though HMNSs are dynamically stable, secular effects such as viscosity or magnetic fields tend to bring HMNSs into uniform rotation and thus lead to collapse. We use our new code, which solves the Einstein-Maxwell-MHD system of equations, to simulate the evolution of magnetized HMNSs. We find that magnetic braking and the magnetorotational instability both contribute to the eventual collapse of HMNSs to rotating black holes surrounded by massive, hot accretion tori and collimated magnetic fields. Such hot tori radiate strongly in neutrinos, and the resulting neutrino-antineutrino annihilation could power short-hard GRBs. Coincident detection of a gravitational wave inspiral and merger signal followed by a short GRB would provide a stringent test of this model.