Researchers have linked periodic pulses in neutron stars to internal disturbances influenced by superfluid vortices. A new model suggests that these disturbances follow a power-law pattern observed in various natural phenomena. Credit: SciTechDaily.com
A recent study has revealed the origins of the mysterious “heartbeats” observed in neutron stars, linking them to disturbances caused by the dynamics of superfluid vortices.
The researchers found that these problems follow a power-law distribution similar to other complex systems and developed a model based on quantum vortex networks that aligns with the observed data without additional tuning.
Discovering the heartbeats of neutron stars
The code for blinking stars in Netflix’s “The Three-Body Problem” may be the stuff of science fiction. However, a new study has deciphered the erratic flickering of neutron stars, revealing the twisted origins of these dead stars’ mysterious “heartbeats.”
When neutron stars—the ultra-dense remnants of massive stars that exploded as supernovae—were first discovered in 1967, astronomers thought their strange periodic pulsations might be signals from an alien civilization. While we now know that these “heartbeats” come from beams of radiation from stellar corpses, not from alien life, their precision makes them excellent cosmic clocks for studying astrophysical phenomena, such as the rotation rates and internal dynamics of celestial bodies.
Sometimes, however, their clockwork mechanism
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>accuracy The pulse propagation time is disrupted by pulses that inexplicably arrive earlier, signaling a glitch or sudden acceleration in the rotation of neutron stars. Although their exact causes remain obscure, the energies of the glitches have been observed to follow the power law (also known as the scaling law)—a mathematical relationship that is reflected in many complex systems, from wealth inequality to patterns in the frequency and magnitude of earthquakes. Just as small earthquakes occur more frequently than larger ones, low-energy glitches are more common than high-energy ones in neutron stars.
The image shows the quantum vortex network model proposed by the study authors. The inner core of the p-wave (pink) surrounds the outer core of the s-wave (gray). Credit: Muneto Nitta and Shigehiro Yasui
By reanalyzing 533 updated datasets from observations of rapidly rotating neutron stars, called pulsars, a team of physicists found that their proposed quantum vortex network naturally aligns with calculations of the power-law behavior of glitch energies without requiring additional tuning, unlike previous models. Their findings are published in the journal
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>Scientific Reports.
Superfluid vortices take a new turn
“More than half a century has passed since the discovery of neutron stars, but the mechanism behind the glitches is still not understood. So we proposed a model to explain this phenomenon,” said the study’s corresponding author, Muneto Nitta, a specially appointed professor and co-principal investigator at the International Institute for Sustainability with Knotted Chiral Metamatter (WPI-SKCM) at Hiroshima University.2).
3D configuration of the quantum vortex network. Credit: Muneto Nitta and Shigehiro Yasui
Previous studies have proposed two main theories to explain these anomalies: starquakes and superfluid vortex avalanches. While starquakes, which behave like earthquakes, could explain the observed power-law pattern, they could not explain all types of anomalies. Superfluid vortices are the most widely cited explanation.
“In the standard scenario, researchers consider that an avalanche of unpinned vortices could explain the origin of the problems,” Nitta said.
However, there is no consensus on what could trigger catastrophic avalanches in vortices.
Key information on neutron star dynamics
“If there were no snagging, it would mean that the superfluid would release the vortices one by one, allowing for a smooth adjustment of the rotation speed. There would be no avalanches or problems,” Nitta said.
“But in our case, we didn’t need any fixation mechanism or additional parameters. We just had to take into account the structure of p- and s-wave superfluids. In this structure, all the vortices are connected to each other in each group, so they can’t be released one by one. Instead, the
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>neutron star must release a large number of vortices simultaneously. This is the key point of our model.
Top view of a quantum vortex network. Credit: Muneto Nitta and Shigehiro Yasui
While the superfluid core of a neutron star rotates at a constant rate, its ordinary component decreases its rotation speed by releasing
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>gravitational waves and electromagnetic pulses. Over time, their speed difference increases, so that the star expels superfluid vortices, which carry a fraction of the angular momentum, to regain its equilibrium. However, when the superfluid vortices intertwine, they drag other superfluid vortices along with them, which explains the problems.
Twisted Clusters and Real-World Data Alignment
To explain how vortices form twisted clusters, the researchers proposed the existence of two types of superfluids in neutron stars. S-wave superfluidity, which dominates the relatively quieter environment of the outer core, favors the formation of integer-quantized vortices (IQVs). In contrast, P-wave superfluidity, which prevails in the extreme conditions of the inner core, favors half-quantized vortices (HQVs). As a result, each IQV in the S-wave outer core splits into two HQVs upon entering the P-wave inner core, forming a cactus-like superfluid structure known as a boojum. As more HQVs separate from the IQVs and connect via boojums, the dynamics of the vortex clusters become increasingly complex, much like cactus arms growing and intertwining with neighboring branches, creating intricate patterns.
The researchers performed simulations and found that the exponent of the power-law behavior of pip energies in their model (0.8±0.2) closely matched the observed data (0.88±0.03). This indicates that their proposed framework accurately reflects real-world neutron star pips.
“Our argument, although simple, is very powerful. Even though we cannot directly observe the p-wave superfluid inside, the logical consequence of its existence is the power-law behavior of the cluster sizes obtained from simulations. Translating this into a corresponding power-law distribution for the pip energies showed that it matches the observations,” said co-author Shigehiro Yasui, a postdoctoral researcher at WPI-SKCM2 and associate professor at Nishogakusha University.
“A neutron star is a very special situation because the three fields of astrophysics, nuclear physics, and condensed matter physics meet at one point. It is very difficult to observe it directly because neutron stars exist far away from us, so we need to establish a deep connection between the interior structure and some observational data of the neutron star.” Reference: “Pulsar glitches from quantum vortex networks” by Giacomo Marmorini, Shigehiro Yasui, and Muneto Nitta, 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 Natural Science Research and Education Center at Keio University. Giacomo Marmorini of the Department of Physics at Nihon University and Aoyama Gakuin University also collaborated on the study.