Neutron star mergers are energetic and ephemeral cosmic events, produced by the cataclysmic collision of supergiant stellar remnants. These exotic events are still the hot topic among astrophysicists globally. These violent cosmological events produce a single, very rapidly rotating compact object. Large observatories such as LIGO, the Einstein Telescope, and our future Cosmic Explorer will be able to detect them. New findings by researchers from the University of Illinois Urbana-Champaign and the University of Valencia elucidate the details of messy mergers of neutron stars. The study emphasizes the effect of strong magnetic fields on oscillation frequencies in such post-merger stars.
For decades, researchers have known that neutron stars exhibit unusual thermodynamic behavior. These properties and more are described by a theoretical formalism known as the equation of state (EoS). The relation between these properties and the extreme conditions produced in a merger remains elusive. This new research shows that even during the merger process, magnetic fields can get extremely amplified. Such fields can be over a billion times stronger than the strongest fields produced on Earth. This amplification poses deep implications for our interpretation of the resulting gravitational wave signals.
Neutron Star Mergers: A Cosmic Laboratory
Whenever neutron stars collide, a new cosmic laboratory is formed, where matter exists in unexplored states that defy our current understanding of science. The equation of state—which describes the physical properties of matter in all conditions—determines how matter behaves at these super-dense extremes. It’s only the first piece of a very big puzzle. In particular, neutron stars are characterized by two main aspects that intrigue researchers: their unique thermodynamic properties and their powerful magnetic fields.
In 2017, the LIGO observatory recorded the first binary neutron star merger. This detection, subsequently confirmed by the associated gamma-ray bursts, represented a historic epoch in astrophysics. This remarkable occurrence attracted significant attention to the field of gravitational wave detection. It unveiled wonderful new prospect of learning even more about neutron stars’ properties.
“The simultaneous detection in 2017 of gravitational waves by LIGO and a gamma-ray burst by NASA satellites from the same cosmic source marked the first time a binary neutron star merger was identified,” – Professor Stuart L. Shapiro
Scholars are currently researching these unusual phenomena. Broadly, they’re interested in the effects strong magnetic fields may have on altering these oscillation frequencies in post-merger neutron stars. Understanding these shifts is important for making accurate inferences from gravitational wave data.
The Role of Magnetic Fields in Oscillation Frequencies
Illinois and Valencia researchers recently tested the new model in a series of statewide simulations. They looked at the influence of different magnetic field topologies on oscillation frequencies post neutron star mergers. Specifically, they took a range of neutron star masses and two different equations of state into account when analyzing the post-merger results of their simulations.
One critical finding indicates that strong magnetic fields significantly shift oscillation frequencies, complicating interpretations of gravitational wave data from neutron star mergers. This complexity is being introduced because the large, next-generation gravitational wave detectors will soon be sensitive to high-frequency signals from these events.
“Next-generation gravitational wave observatories, like Cosmic Explorer, will be able to detect the actual merger of two neutron stars as they form a single rotating compact object and the various frequencies of oscillations associated with the merger process,” – Antonios Tsokaros
The implications of these findings are substantial. As the authors point out, prior studies may have overlooked significant impacts of magnetic fields. Such effects can have strong impact on the thermodynamic properties in neutron stars. In this way, past studies might have led to incorrect conclusions regarding the surprising behavior of matter at extreme conditions. That was the case because it didn’t fully account for fundamental driving forces.
“Previous work by other investigators has been overly optimistic in trying to identify the thermodynamic properties in the interior of the neutron stars by completely ignoring the effects that come from its magnetic field. On the other hand, we explicitly show that this omission can be misleading and that the magnetic field should be included for the correct interpretation of the observations,” – Antonios Tsokaros
Future Directions and Research Implications
As our research team further investigates this intricate landscape, preparations for additional simulations at even finer resolutions are already in the works. Until now, evolving computational limitations have prevented in-depth analyzes. Today, breakthroughs in technology and methodology combine to enable us to do more sophisticated analyses.
Jamie Bamber, the lead researcher on this study, explained why it’s vital to know exactly where these magnetic fields are.
“The magnetic field is amplified to large values during the merger,” – Jamie Bamber
More in-depth studies on these topics are in the works. Collaborating with theoretical astrophysicists, the researchers hope to better understand the role of magnetic fields relative to other forces at play during and after neutron star mergers. Continuing efforts like this will improve our understanding of gravitational wave data. It would allow for penetrating new discoveries into the nature of matter at its most fundamental in exotic environments.