Rest mass density (upper panels) and angular speed (lower panels) on the equatorial plane for an undeveloped analog (left column) and the magnetized analog (right column), 10 ms after the fusion. White and black lines mean the contours of density. Two main differences between magnetized and not magnetized cases are 1) at a given moment, the rest magnetized is more axisymmetrical (more circular equatorial contours) than that unaggerticized, and 2) the angular speed in the heart of the rest is higher and more uniform in the magnetized case than in the non -magnetized nucleus. Credit: Tsokaros et al.
Neutron star mergers are collisions between neutron stars, collapsed kernels of what was once massive supergiante stars. These mergers are known to generate gravitational waves, waves carrying energy spreading through a gravitational field, which emerges from the acceleration or disturbance of a massive body.
The collisions between neutron stars have been the subject of numerous studies of theoretical physics, because a more in -depth understanding of these events could give interesting information on the behavior of matter to extreme densities. The behavior of matter to extremely high densities is currently described by a theoretical framework known as the State equation (EOS).
Recent research in astrophysics has explored the possibility that the characteristics of the EOS, such as phase transitions or a crossover crossing, can be deduced from the gravitational wave spectrum observed after the stars of neurons have merged. However, most of this theoretical work did not take into account the effects of magnetic fields on this spectrum.
Researchers from the University of Illinois Urbana-Champaign and the University of Valence recently made a series of simulations aimed at better understanding the impact of magnetic fields on the oscillating frequencies of post-fusion stars. Their article, published in Physical examination lettersshows that magnetic fields alone can also cause frequency changes, therefore the interpretation of the fusion observations of neutron stars could be more difficult than expected previously.
“The observatories of new generation gravitational waves, such as Cosmic Explorer, will be able to detect the real fusion of two neutron stars because they form a single rotary compact object and the various frequencies of oscillations associated with the fusion process,” said Antonios Tsookaros, the main author of the article, to Phys.org.
“These frequencies code for many of the characteristics of neutron stars. Therefore, identifying them correctly will allow us to understand many properties still unknown of these extraordinary objects.”
Neutron stars have two main characteristics which must still be fully understood and make them fascinating of physical laboratories. First, they have unique thermodynamic properties, such as those described by EOS, in its heart. 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 of more than a billion times higher than the largest magnetic field ever created by humans.
The lower layout shows the frequency shift as a function of the amplitude of the magnetic field for the main oscillation mode compared to the undeveloped case. Such a frequency discrepancy in a remaining fusion remains can be caused by a certain number of reasons: 1) the existence of a phase transition or more generally, the presence of an abnormal and non -convex dynamic. 2) The existence of a Quark-Hadron Crossing State equation. 3) Finished temperature effects. 4) The rigidity of the state equation. 5) Effects out of balance, such as loose viscosity. 6) The magnetic field. 7) The rotation of the stars with previous neutron. In addition to articles 6, 7 in the list above, everything else is linked to the unknown State equation, again, in its cold or in its hot sector. The extent of the expected discrepancies varies for each of the reasons above, but overlap is significant. This means that any of the changes predicted by elements 1 to 5 can be masked by the magnetic field (or even the anterior spin of the neutron star), and therefore any interpretation of the observation data must be carried out with caution. Credit: Tsokaros et al.
“Our work systematically tries to understand the effect of the magnetic field on the oscillating frequencies of the post-fusion neutrons and to inform about various competing effects,” said Tsokaros. “The previous work of other researchers has been too optimistic by trying to identify the thermodynamic properties inside neutron stars by completely ignoring the effects that come from its magnetic field. On the other hand, we show explicitly that this omission can be misleading and that the magnetic field should be included for the correct interpretation of observations.”
As part of their recent study, Tsokaros and his colleagues carried out general simulations of relativistic magnetohydrodynamics to explore the effects of magnetic fields on the oscillating frequencies of post-fusion stars. In these simulations, they used two neutron stars, two masses of different neutron stars and three different magnetic field topologies.
“The magnetic field is amplified to large values during the fusion,” said Jamie Bamber, a post-doctoral student working with Tsokaros and Shapiro teachers. “Our simulations have shown that the strong magnetic field oscillates the rest of the fusion and produces gravitational waves at a higher frequency. This increase in frequency can hide changes in the frequency of a different origin such as a change in EOS, which makes the interpretation of possible observations more complicated than before.”
Professor Milton Ruiz added: “To make a precise assessment of the post-fusion phase in the binary neutron stars mergers, it is therefore necessary to include the effects of the magnetic field. Not to do so can lead to erroneous conclusions on the physical properties of the system.”
Overall, this recent study suggests that the effects of magnetic fields could complicate the interpretation of gravitational waves from neutron stars. In their future research, Tsokaros and his colleagues plan to corroborate their recent results by performing other simulations to even higher resolutions which were previously prohibitive on the calculation.
“The simultaneous detection in 2017 of gravitational waves by Ligo and a gamma radiography exploded by satellites from NASA from the same cosmic source marked the first time that a fusion of binary neutron stars was identified,” said Professor Stuart L. Shapiro.
“This has marked a breakthrough in multi-meprising astronomy and triggered simulations in relativistic magnetohydrodynamics like those we have carried out at the University of Illinois. However, many signature characteristics of these simulations will only be identified by the next generation of detectors of severity of binary neutron stars.”
More information:
Antonios Tsokaros et al, masking the effects of the state equation in the mergers of binary neutron stars, Physical examination letters (2025). DOI: 10.1103 / Physrevlett.134.121401. On arxiv: DOI: 10.48550 / Arxiv.2411.00939
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