A new study has revealed the twisted origin of the mysterious ‘arrhythmias’ that occur in the pulses emitted by neutron stars.
When these ultra-dense 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 “heartbeats” originate from beams of radiation from stellar corpses, not from extraterrestrial 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 clock accuracy is thrown off by pulses that inexplicably arrive earlier, indicating a glitch or a sudden acceleration in the neutron star’s spins. Although its exact causes remain unclear, failure energies have been observed to follow the power law (also known as scaling law), a mathematical relationship that is reflected in many complex systems, from wealth inequality to frequency-magnitude patterns in earthquakes. Just as smaller earthquakes occur more frequently than larger ones, low-energy faults are more common than high-energy faults in neutron stars.
By reanalyzing 533 updated data sets of observations of rapidly spinning neutron stars, called pulsars, a team of physicists found that their proposed quantum vortex network naturally aligns with calculations about the power-law behavior of fault energies without the need for additional adjustments, unlike previous models. Their findings are published in the journal Scientific Reports.
“More than half a century has passed since the discovery of neutron stars, but the mechanism by which failures occur is still not understood. That is why we proposed a model to explain this phenomenon,” said study corresponding author Muneto Nitta. , specially appointed professor and co-principal investigator of the International Institute for Sustainability with Knotted Meta-Chiral Matter (WPI-SKCM2) at Hiroshima University.
Previous studies have proposed two main theories to explain these faults: 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 faults. Superfluid vortices are the most invoked explanation.
“In the standard scenario, researchers believe that unbound vortex avalanches could explain the origin of the faults,” Nitta said.
However, there has been no consensus on what could trigger a catastrophic onslaught of vortices.
“If there were no pinning, it would mean that the superfluid releases vortices one by one, allowing smooth adjustment in rotation speed. There would be no avalanches or failures,” Nitta said.
“But in our case, we didn’t need any pinning mechanism or additional parameters. We only needed to consider the structure of p-wave 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 neutron star has to release a large number of vortices simultaneously. That’s the key point of our model.”
While the superfluid core of a neutron star rotates at a constant rate, its ordinary component slows its rotation speed, releasing gravitational waves and electromagnetic pulses. Over time, the discrepancy between their velocities increases, so the star expels superfluid vortices, carrying a fraction of the angular momentum, to regain equilibrium. However, as the superfluid vortices become entangled, they drag others with them, which explains the glitches.
To explain how vortices form twisted clusters, researchers proposed the existence of two types of superfluids in neutron stars. S-wave superfluidity, which dominates the relatively more docile 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 semiquantized vortices (HQV).
As a result, each IQV in the outer s-wave core splits into two HQVs upon entering the inner p-wave core, forming a cactus-like superfluid structure known as a boojum. As more HQV separate from IQV and connect via boojums, the dynamics of vortex clusters become increasingly complex, much like the arms of cacti sprouting and intertwining with neighboring branches. , creating intricate patterns.
The researchers ran simulations and found that the exponent of the power-law behavior of the fault energies in their model (0.8 +/- 0.2) closely matched the observed data (0.88 +/- 0.03). This indicates that the proposed framework accurately reflects real-world neutron star faults.
