BINARY NEUTRON STAR MERGERS: A JET ENGINE FOR SHORT GAMMA-RAY BURSTS

Area of Science:

• Astrophysics
• Computational Physics
• General Relativity

Background:

• Binary neutron star mergers are key astrophysical events with observable electromagnetic counterparts.
• The role of magnetic fields in the dynamics and outflows of neutron star mergers is not fully understood.
• Magnetohydrodynamics in full general relativity (GRMHD) is crucial for simulating these extreme environments.

Purpose of the Study:

• To investigate the impact of magnetic field geometry on binary neutron star merger outcomes using GRMHD simulations.
• To determine if jets can be launched from the hypermassive neutron star and accretion disk post-merger.
• To compare jet launching mechanisms in neutron star-neutron star mergers versus black hole-neutron star mergers.

Main Methods:

• Performed GRMHD simulations of quasi-circular, equal-mass, irrotational binary neutron stars.
• Explored two magnetic field configurations: exterior-penetrating dipole fields and interior-confined dipole fields.
• Analyzed the post-merger evolution, including hypermassive neutron star formation, black hole collapse, and accretion disk dynamics.

Main Results:

• The merger consistently resulted in a hypermassive neutron star that collapsed into a black hole surrounded by a magnetized accretion disk.
• A collimated, mildly relativistic outflow (incipient jet) was launched from the black hole pole region approximately 4000M after merger.
• Incipient jets were observed in both magnetic field configurations, including when the field was initially confined to the stellar interiors.

Conclusions:

• Magnetized accretion disks around black holes formed after neutron star mergers can launch powerful jets.
• The confinement of the initial magnetic field within the neutron stars does not preclude jet formation.
• These findings have implications for understanding the electromagnetic counterparts of neutron star mergers and the production of relativistic outflows.