Revealing the Twisted Roots of Mysterious Neutron Star Pulses

A recent study revealed the origin of the mysterious “hearts” observed in neutron stars and related them to glitches caused by the dynamics of superfluid vortices.

The researchers found that these defects follow a power-law distribution similar to other complex systems, and developed a model based on quantum vortex networks that is consistent with the observed data without further tuning.

Discovering the heartbeats of neutron stars

The asterisks flashing code in Netflix’s “3 Body Problem” may be science fiction. But a new study has unraveled the erratic flickering of neutron stars and revealed the twisted origins of the mysterious “hearts” of these dead stars.

When neutron stars—the ultradense remnants of massive stars that exploded in supernovae—were first discovered in 1967, astronomers thought their strange periodic pulses might be signals from an extraterrestrial civilization. Although we now know that these “hearts” come from the radiation beams of stellar corpses, not extraterrestrial life, their precision makes them excellent cosmic clocks for studying astrophysical phenomena such as the rotation rates and internal dynamics of celestial bodies.

However, sometimes their hours accuracy it is disrupted by pulses that inexplicably arrive earlier and signal a malfunction or sudden acceleration of neutron star rotations. While their exact causes remain unclear, fault energies have been observed to obey a power law (also known as a scaling law)—a mathematical relationship reflected in many complex systems from wealth inequality to frequency-magnitude patterns in earthquakes. Just as smaller earthquakes occur more often than larger ones, low-energy faults are more common in neutron stars than high-energy ones.

A team of physicists reanalyzed 533 current data sets from observations of rapidly rotating neutron stars, called pulsars, and found that their proposed quantum vortex network naturally matches power-law calculations of glitch energy behavior without the need for further tuning, unlike past models. Their findings are published in a journal Scientific reports.

Superfluid vortices get a new twist

“More than half a century has passed since the discovery of neutron stars, but the mechanism by which the perturbations occur is still unknown. So we proposed a model to explain this phenomenon,” said the study’s corresponding author Muneto Nitta, specially appointed professor and co-investigator at the International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM) of Hiroshima University.²).

Previous studies have proposed two main theories to explain these glitches: star shocks and superfluid vortex avalanches. While stellar shocks that behave like earthquakes could explain the observed power-law pattern, they could not explain all types of faults. A widely used explanation is superfluid vortices.

“In the standard scenario, the researchers believe that an avalanche of unpinned vortices could explain the origin of the glitches,” Nitta said.

However, there was no consensus on what could trigger a catastrophic vortex avalanche.

Key insights into neutron star dynamics

“If there was no pinning, it means the superfluid is releasing the vortices one by one, allowing for a smooth adjustment of the spin speed. There would be no avalanches and no glitches,” Nitta said.

“But in our case, we didn’t need any pinning mechanism or other parameters. It was enough to consider the structure of superfluid pa s waves. In this structure, all the vortices in each cluster are interconnected, so they cannot be released one by one. Instead, neutron star must release a large number of vortices at the same time. That’s the key point of our model.”

While the superfluid core of a neutron star rotates at a constant rate, its normal component slows down its rotation rate by releasing gravitational waves and electromagnetic pulses. Over time, their velocity difference increases, so the star ejects superfluid vortices that carry a fraction of the angular momentum to regain equilibrium. However, as superfluid eddies are entangled, they drag others along, which explains the glitches.

Twisted Clusters and Real-World Data Alignment

To explain how vortices form twisted clusters, scientists have proposed the existence of two types of superfluids in neutron stars. The S-wave superfluidity that dominates the relatively tamer environment of the outer core favors the formation of integer quantized vortices (IQVs). In contrast, the p-wave superfluidity prevailing 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 s-wave inner core, forming a cactus-like superfluid structure known as a boojum. As more HQVs separate from IQVs and connect via boojums, the dynamics of the vortex clusters become increasingly complex, much like cactus arms sprouting and entwining with neighboring branches to form complex patterns.

The researchers ran simulations and found that the exponent for the power-law behavior of glitch energies in their model (0.8 ± 0.2) closely matched the observed data (0.88 ± 0.03). This suggests that their proposed framework accurately reflects real neutron star defects.

“Our argument, while simple, is very powerful.” Although we cannot directly observe the p-wave superfluid inside, a logical consequence of its existence is the power-law behavior of the cluster sizes obtained from the simulations. Converting this to the corresponding power law distribution for the glitch energies showed that it matches the observations,” said co-author Shigehiro Yasui, a postdoctoral researcher at WPI-SKCM.² and adjunct professor at Nishógakusha 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 directly because neutron stars exist far away from us, so we have to make a deep connection between the internal structure and some observational data from 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 Keio University Department of Physics and Research and the Natural Science Education Center. Another collaborator on the study is Giacomo Marmorini from the Department of Physics of Nihon University and Aoyama Gakuin University.