Study explores the effects of magnetic fields on their oscillating frequencies

Neutron star mergers are collisions between neutron stars, the collapsed cores of what were once massive supergiant stars. These mergers are known to generate gravitational waves, energy-carrying waves propagating through a gravitational field, which emerge from the acceleration or disturbance of a massive body.

Collisions between neutron stars have been the topic of many theoretical physics studies, as a deeper understanding of these events could yield interesting insights into how matter behaves at extreme densities. The behavior of matter at extremely high densities is currently described by a theoretical framework known as the equation of state (EoS).

Recent astrophysics research has explored the possibility that EoS features, such as phase transitions or a quark-hadron crossover, could be inferred from the gravitational wave spectrum observed after neuron stars have merged. However, most of these theoretical works did not consider the effects of magnetic fields on this spectrum.

Researchers from the University of Illinois Urbana-Champaign and the University of Valencia recently performed a series of simulations aimed at better understanding the impact of magnetic fields on the oscillating frequencies of post-merger neutron stars. Their paper, published in Physical Review Letters, shows that magnetic fields alone can also result in frequency shifts, thus interpreting neutron star merger observations could be more challenging than previously anticipated.

“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, lead author of the paper, told Phys.org.

“These frequencies encode many of the characteristics of the neutron stars. Therefore, identifying them correctly will enable us to understand many of the yet unknown properties of these extraordinary objects.”

Neutron stars have two main characteristics that are yet to be fully understood and make them fascinating physical laboratories. Firstly, they possess unique thermodynamic properties, such as those described by the EoS, in its core. Due to these properties, just a spoonful of neutron star material weighs as much as Mount Everest.

The other key characteristic of neutron stars is their magnetic field. During neutron star mergers, this magnetic field can reach values over a billion times higher than the largest magnetic field ever created by humans.

“Our work tries systematically to understand the effect of the magnetic field on the oscillating frequencies of the post-merger neutron star and to inform about various competing effects,” said Tsokaros. “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.”

As part of their recent study, Tsokaros and his colleagues performed general relativistic magnetohydrodynamics simulations to explore the effects of magnetic fields on the oscillating frequencies of post-merger neutron stars. In these simulations, they used two neutron star EoSs, two different neutron star masses and three different magnetic field topologies.

“The magnetic field is amplified to large values during the merger,” explained Jamie Bamber, a Postdoc working with Professors Tsokaros and Shapiro. “Our simulations showed that the strong magnetic field causes the merger remnant to oscillate and produce gravitational waves at a higher frequency. This increase in frequency can mask frequency shifts from a different origin such as a change in the EoS, making the interpretation of possible observations more complicated than previously thought.”

Professor Milton Ruiz added, “To make an accurate assessment of the post-merger phase in binary neutron star mergers, one thus needs to include the effects of the magnetic field. Failing to do so may lead to erroneous conclusions about the physical properties of the system.”

Overall, this recent study suggests that the effects of magnetic fields could complicate the interpretation of gravitational wave data originating from neutron star mergers. In their future research, Tsokaros and his colleagues plan to corroborate their recent results by performing further simulations at even higher resolutions that were previously computationally prohibitive.

“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,” said Professor Stuart L. Shapiro.

“This marked a breakthrough in multi-messenger astronomy and has triggered simulations in relativistic magnetohydrodynamics like the ones we have been performing at the University of Illinois. Yet many of the signature features of these simulations will only be identified by the next generation of gravitational wave detectors, like the Einstein Telescope and Cosmic Explorer, which will detect the high frequencies associated with the merger and post-merger of binary neutron stars.”

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