Recent research has uncovered the origin of the mysterious “heartbeats” observed in neutron stars, linking them to malfunctions caused by the dynamics of superfluid vortices.
The researchers found that these glitches follow a power-law distribution, similar to other complex systems, and developed a model based on quantum vortex networks that matches the observed data without any additional tuning.
Discovering the heartbeat of a neutron star
The blinking star code in Netflix’s “3 Body Problem” may be science fiction, but a new study has decoded the erratic blinking of neutron stars, shedding light on the origins of these dead stars’ mysterious “heartbeats.”
When neutron stars – the super-dense remnants of massive stars that have exploded as supernovae – were first discovered in 1967, astronomers thought their strange, periodic pulsations might be signals from extraterrestrial civilisations. We now know that these “beats” don’t come from extraterrestrial life, but from radiation beams from dead stars, but their precision makes them excellent cosmic clocks for studying astrophysical phenomena such as the rotation speeds and internal dynamics of celestial objects.
But sometimes their clockwork Accuracy The pulse’s mysteriously early arrival indicates a glitch or sudden acceleration in the neutron star’s spin. The exact cause is unclear, but the glitch energy is thought to be Power Law (Also known as a scaling law) A mathematical relationship reflected in many complex systems. Wealth inequality To Frequency-Amplitude Pattern Just as small earthquakes occur more frequently than large ones, low-energy glitches are more common in neutron stars than high-energy ones.
The team of physicists reanalyzed 533 recent data sets from observations of rapidly spinning neutron stars called pulsars and found that, unlike past models, the quantum vortex network they propose naturally matches calculations of the power law behavior of the glitch energy without requiring any additional tuning. Their findings are published in the journal Nature. Scientific Reports.
New developments in superfluid vortices
“More than half a century has passed since the discovery of neutron stars, but the mechanism by which the glitch occurs has still not been clarified. Therefore, we proposed a model to explain this phenomenon,” said Muneto Nitta, a project professor at Hiroshima University’s International Institute for Sustainability Studies (WPI-SKCM) and co-principal investigator and corresponding author of the study.2).
Previous studies have proposed two main theories to explain these glitches: starsquakes and superfluid vortexes. Starquakes, which behave like earthquakes, could explain the power law patterns observed, but cannot explain all types of glitches. Superfluid vortexes are the most widely cited explanation.
“In the standard scenario, the researchers believe that an avalanche of unanchored vortices could explain the cause of the glitch,” Nitta said.
But there’s no consensus on what triggers the vortex and creates the devastating avalanche.
Important insights into neutron star dynamics
“Without pinning, the superfluid would be able to shed the vortices one by one and smoothly adjust the rotation speed — no avalanches or malfunctions would occur,” Nitta said.
“But in our case, we didn’t need any pinning mechanism or additional parameters. We just had to consider the structure of the p- and s-wave superfluid, in which all the vortices are connected to each other within each cluster, so they can’t be released one by one. Instead, Neutron Star “You need to release a lot of vortices at the same time, and that’s the key point of our model.”
The superfluid core of a neutron star rotates at a constant speed, but the normal components are released to slow it down. Gravitational waves And then the electromagnetic pulse. Over time, that difference in speed gets so great that the star can release superfluid vortices that carry some of the angular momentum back to balance. But when the vortices get tangled, they drag other vortices along with them, and that’s when things go wrong.
Aligning Twisted Clusters with Real-World Data
To explain how the vortices form the twisted clusters, the researchers proposed that two types of superfluids exist in neutron stars. S-wave superfluidity, which dominates the relatively benign environment of the outer core, promotes the formation of integer quantized vortices (IQVs). In contrast, p-wave superfluidity, which prevails in the extreme conditions of the inner core, promotes the formation of half-quantized vortices (HQVs). As a result, each IQV in the s-wave outer core splits into two HQVs as it enters the p-wave inner core, forming a cactus-like superfluid structure called a boojum. As more HQVs split off from the IQVs and connect through boojums, the dynamics of the vortex clusters become increasingly complex, much like the arms of a cactus sprouting and intertwining with neighboring branches to form intricate patterns.
The researchers ran simulations and found that the exponent of the power-law behavior of the glitch energy in their model (0.8±0.2) closely matches the observational data (0.88±0.03), indicating that the proposed framework accurately reflects real-world neutron star glitches.
“Our argument is simple but very powerful. Although we cannot directly observe the inner p-wave superfluid, a logical consequence of its existence is the power-law behavior of the cluster size we obtained from our simulations. When we convert this into a corresponding power-law distribution of glitch energies, we find that it agrees with the observations,” said co-author Shigehiro Yasui, a postdoctoral researcher at WPI-SKCM.2 Associate Professor at Nishogakusha University.
“Neutron stars are a very special situation where three fields come together: astrophysics, nuclear physics, and condensed matter physics. Neutron stars are very difficult to observe directly because they are so far away from us, so we need to make a deep connection between the internal structure of neutron stars and the observational data.” References: Giacomo Marmorini, Shigehiro Yasui, and Muneto Nitta, “Pulsar Glitches from Quantum Vortex Networks,” April 3, 2024, Scientific Reports.
DOI: 10.1038/s41598-024-56383-w
Yasui and Nitta are also affiliated with the Department of Physics and the Center for Research and Education in Natural Sciences at Keio University. Another collaborator on this work is Giacomo Marmorini of the Department of Physics at Nihon University and Aoyama Gakuin University.