Monster Shocks from Collapsing Magnetars

The collapse of an extremely dense, highly magnetized stellar remnant into a black hole is an extreme event. New simulations explore what happens when a magnetar collapses into a black hole, providing a potential connection between these events and gamma-ray bursts.

Becoming a Black Hole

When massive stars die, they collapse in upon themselves and can leave behind their condensed cores in the form of neutron stars. For many stars, the journey ends here: over time, neutron stars slowly cool, spin down, and fade. Others have a different final destination: if one neutron star collides with another, the remnant of the merger can collapse further, becoming a black hole.

This process can be nearly instantaneous, the merger remnant shrinking down to a black hole in just milliseconds. Sometimes, though, the merger remnant survives for several hours, coalescing into a rapidly spinning, highly magnetized neutron star called a magnetar. As the magnetar sheds energy through its outflowing magnetized wind, it’s no longer able to support itself, and it collapses into a black hole. What a distant observer might see when this happens is not yet clear.

Waves, Shocks, and Rays

To understand what happens during a magnetar’s collapse, Elias Most (California Institute of Technology) and collaborators turned to complex general relativistic magnetohydrodynamic simulations. Magnetar collapse been modeled before, but this study improves upon previous work by tackling a full magnetohydrodynamic approach to the problem, allowing the team to investigate the role that shocks play during and after the collapse. Because astrophysical shocks provide a way to accelerate particles and generate high-energy radiation, a powerful shock from a collapsing magnetar could be a source of gamma-ray bursts: brief, powerful flashes of gamma rays of unknown origin.

The team modeled a rotating magnetar with a surface magnetic field strength of 10¹⁶ Gauss, about 15 quadrillion times stronger than Earth’s magnetic field. After introducing a perturbation into the system, the magnetar collapsed, forming a black hole and launching a near-lightspeed wave into the magnetar’s magnetosphere. The wave then accelerated plasma trapped in the magnetosphere, launching a shock. The so-called “monster shock” generated by the collapse of a magnetar may be the strongest shock in the universe.

Bundles of Bursts

The team’s simulations showed that a hot, powerful, magnetized electron–positron plasma outflow explodes outward in the wake of the monster shock. As they hurry outward, the electron–positron pairs collide, annihilating one another and releasing gamma rays. This flash of gamma rays likely lasts a few milliseconds, similar to the duration of many gamma-ray bursts.

To make this possibility even more interesting, Most and collaborators showed that the newborn black hole’s ring-down — a fleeting period in which the event horizon of the black hole vibrates — would imprint slight variability on the gamma-ray signal. This could explain the tiny variations seen in some gamma-ray bursts.

Not only does this scenario predict the formation of a powerful gamma-ray burst, it might also produce another type of mysterious astrophysical signal: a fast radio burst. This connection is only tenuous, as Most’s team notes that plasma surrounding the collapsed magnetar would likely swallow the radio signal before it escaped. Only under certain conditions could a fast radio burst or a gamma-ray burst escape the plasma’s grasp and make its way to our telescopes.

Citation

“Monster Shocks, Gamma-Ray Bursts, and Black Hole Quasi-normal Modes from Neutron-Star Collapse,” Elias R. Most et al 2024 ApJL 974 L12. doi:10.3847/2041-8213/ad7e1f