Inside a neutron star – new insights from gravitational waves

We might start to see atoms interacting with each other in ways "we have not yet seen."
Baba Tamim
Two merging neutron stars in the galaxy NGC 4993.
Two merging neutron stars in the galaxy NGC 4993.

NASA's Goddard Space Flight Center/CI Lab 

University of Birmingham researchers have demonstrated how unique vibrations, which are caused by interactions between the two stars' tidal fields as they approach each other, affect gravitational-wave observations.

Taking these movements into account could significantly improve our understanding of the data collected by the Advanced LIGO and Virgo instruments, according to a press release published on the institute's official website on Thursday.

"Scientists are now able to get lots of crucial information about neutron stars from the latest gravitational wave detections," said Dr. Geraint Pratten of the University of Birmingham's Institute for Gravitational Wave Astronomy. "Details such as the relationship between the star's mass and its radius, for example, provide crucial insight into fundamental physics behind neutron stars."

"If we neglect these additional effects, our understanding of the structure of the neutron star as a whole can become deeply biased," added Pratten, the lead author of the paper.

LIGO and Virgo Collaboration

Since the LIGO Scientific Collaboration and the Virgo Collaboration detected the first gravitational waves in 2016, scientists have been working to improve their understanding of the massive collisions that produce these signals, including the physics of a neutron star at supra nuclear densities.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves. And Virgo is a gravitational interferometer, which is a type of telescope designed to detect gravitational waves from gravity-driven astrophysical phenomena.

The instruments were designed to detect gravitational waves, which are ripples in time and space caused by black holes and neutron stars merging.

The scientific team's refinements are the latest contribution from the University of Birmingham to the Advanced LIGO program.

These refinements are really important. "Within single neutron stars, we can start to understand what's happening deep inside the star's core, where matter exists at temperatures and densities we cannot produce in ground-based experiments," said Dr. Patricia Schmidt, co-author of the study and an associate professor at the Institute for Gravitational Wave Astronomy.

"At this point, we might start to see atoms interacting with each other in ways we have not yet seen – potentially requiring new laws of physics."

Hopes for next observation

The researchers hope to have a new model ready for Advanced LIGO's next observing run and even more advanced models for the next generation of Advanced LIGO instruments, known as A+, which will begin their first observing run in 2025.

Since the program's inception, researchers at the University's Institute for Gravitational Wave Astronomy have been heavily involved in the design and development of the detectors. Natalie Williams, a Ph.D. student, is already working on calculations to further refine and calibrate the new models.

The study first appeared in Physical Review Letters.

Study abstract:

Gravitational waves (GWs) from inspiraling neutron stars afford us a unique opportunity to infer the as-of-yet unknown equation of state of cold hadronic matter at supranuclear densities. During the inspiral, the dominant matter effects arise due to the star’s response to their companion’s tidal field, leaving a characteristic imprint in the emitted GW signal. This unique signature allows us to constrain the cold neutron star equation of state. At GW frequencies above ≳ 800 Hz, however, subdominant tidal effects known as dynamical tides become important. In this Letter, we demonstrate that neglecting dynamical tidal effects associated with the fundamental (f) mode leads to large systematic biases in the measured tidal deformability of the stars and hence in the inferred neutron star equation of state. Importantly, we find that f-mode dynamical tides will already be relevant for Advanced LIGO’s and Virgo’s fifth observing run (∼ 2025)—neglecting dynamical tides can lead to errors on the neutron radius of O(1 km), with dramatic implications for the measurement of the equation of state. Our results demonstrate that the accurate modeling of subdominant tidal effects beyond the adiabatic limit will be crucial to perform accurate measurements of the neutron star equation of state in upcoming GW observations.

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