Dancing with the Neutron Stars - Its Electric and Dynamic

The online issue of The Astrophysical Journal: Letters (Vol.981, No.1) features new electrodynamics modeling work by one of Dartmouth's newest assistant professors Jens Mahlmann. Mahlmann is a computational astrophysicist who specializes in simulating extreme plasmas around compact objects, like in neutron star magnetospheres. His publication discusses novel insights on interacting stellar magnetospheres, and how some of these processes can affect astronomical observation.

In his current research, Mahlmann investigates energy release in the interacting magnetospheres of binary neutron stars. Mahlmann worked closely with his advisor, Andrei Beloborodov from Columbia University, to build and analyze global 3D force-free electrodynamics simulations of binary systems. Much of their work was done at the Max-Planck Institute for Astrophysics in Garching, Germany, and on NASA's Pleiades supercomputer. 

Their models revealed that the magnetic field structure in merging binary neutron stars is richer than previously realized. There are different current layers within the binary system's magnetosphere. In the interaction region between the stars, a current layer can—despite being a highly-magnetized shear flow—develop the Kelvin Helmholtz instability. The team also discovered that instabilities in the interaction layer act like a drag, slowing down magnetic field lines that are rotating with the star. As the footpoints anchored to the stellar surface keep rotating, field lines are released from the drag region in a slingshot like motion. These compressive dynamics can launch fast magnetosonic waves into the magnetosphere.

Instabilities in the interaction layer leading to increased dissipation and the emission of fast magnetosonic waves could have important implications for astronomical observations. Researchers can use both to derive limits on X-ray emission and Fast Radio Burst (FRB) production in these systems. Mahlmann's study shows that FRB generation is not easy for merging binary neutron stars, which is consistent with estimates that only a very low percentage of FRBs may originate in binary neutron star systems.

Mahlmann has been working towards this publication since he was a visiting researcher with Dartmouth a year ago. During that time he learned about Professor Yi-Hsin Liu's research on Earth's magnetosphere. Reflecting on these open science discussions in the department, Mahlmann remarks "It is fascinating how observations from near-Earth environments can inspire us to understand the extreme plasmas surrounding compact objects like neutron stars". Mahlmann is excited to continue building his body of collaborative and inclusive scientific research at Dartmouth. His 'Extreme Plasmas around Compact Objects' group's future projects will focus on two key areas to help interpret observational data: investigating FRB wave propagation in magnetically active plasmas and examining magnetospheric instabilities. 

You can read the full publication here. Short videos of Mahlmann's simulations are available to watch here and here.