Scientists may have unlocked the secret of the still-beating hearts of the universe’s most extreme “dead stars,” but the explanation is twisted.
The team thinks that an avalanche of quantum tornadoes is causing this ‘disturbance’ in the rotation of a class of neutron stars called pulsars. This happens when they become entangled with neighbouring stars, like the arms of a row of cacti that are close together, creating twisted and complex patterns.
“More than half a century has passed since the discovery of neutron stars, but the mechanism of why failures occur is still not understood,” team member and professor Muneto Nitta of Hiroshima University said in a statement. “Therefore, we proposed a model to explain this phenomenon.”
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A team of researchers looked at 533 observations of pulsars to solve the mystery of these glitches. They propose that glitches are the result of a “quantum vortex network” that fits power-law calculations, and thus developed a model that does not require “extra tuning,” unlike previous neutron star glitch models.
Neutron star ‘disruptions’ go deep
Neutron stars are born when massive stars die, run out of fuel for nuclear fusion, and collapse under their own gravity. Their outer layers are blown away in enormous supernova explosions. This leaves behind a stellar core with between one and two times the mass of the sun, which is compressed to a diameter of about 12 miles (kilometers). That is small enough to fit inside the average city on Earth.
The result of this collapse is that electrons and protons are squeezed together, creating a sea of neutrons so dense that if a tablespoon were brought to Earth, it would weigh more than 1 billion tons. That’s heavier than Mount Everest.
The crushing of stellar cores is also responsible for the rapid spin of young neutron stars, with some reaching speeds of up to 700 rotations per second. This is due to the conservation of angular momentum, which is similar to a skater on Earth pulling in their arms to increase the speed of their spin.
Recently “dead” neutron stars or “pulsars” appear to pulsate because they shoot out beams of radiation from their poles as they spin rapidly. Pulsars periodically brighten when their beams are pointed directly at Earth, causing them to appear to pulsate (hence their name). This pulsation can be likened to a cosmic “heartbeat” that is so precise that these young neutron stars can be used as cosmic stopwatches in so-called pulsar timing arrays to measure the timing of celestial events.
There is a problem, however. Some neutron stars appear to occasionally “glitch,” briefly speeding up their rotation and the delivery of their pulses, disrupting the regularity of their heartbeats. The cause of these glitches is shrouded in mystery.
Pulsar glitches appear to follow a similar pattern, or “power-law,” as earthquakes on Earth. Just as low-magnitude earthquakes are more common than high-magnitude earthquakes, low-energy glitches are more common in pulsars than high-energy glitches and extreme glitches.
There are two main mechanisms associated with neutron star disruptions: starquakes and tiny quantum vortex “avalanches” that form like microscopic hurricanes in the superfluid soup that makes up a neutron star’s interior.
Quantum vortices are generally more widely accepted as an explanation than starquakes, because while starquakes follow a power law like earthquakes, they struggle to explain all types of neutron star disruptions. Yet, despite their wider acceptance, there is no real explanation for what could cause a catastrophic avalanche of superfluid vortices to reach the surface of a neutron star and cause it to spin faster.
“In the standard scenario, researchers consider that an avalanche of unpinned vortices could explain the origin of glitches,” Nitta explains in the press release. “If there were no pinning, this means that the superfluid releases vortices one by one, allowing for a smooth adjustment of the rotation rate. There would be no avalanches and no glitches.”
Nitta added that the team’s model doesn’t need an additional pinning mechanism. It only needs to account for a structure consisting of two types of waves rippling through the superfluid interior of a neutron star: a “P-wave,” a fast-moving longitudinal wave, and an “S-wave,” a slower-moving transverse wave.
“In this structure, all the vortices in each cluster are connected to each other, so they cannot be released one by one,” Nitta continued. “Instead, the neutron star must release a large number of vortices at once. That is the key point of our model.”
Ordinary matter in neutron stars is an obstacle
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The team’s model suggests that the superfluid core of a neutron star spins at a constant rate, but the non-superfluid “regular” part is stuck to it. The result is the neutron star’s spin rate slowing down by emitting electromagnetic pulses and tiny ripples in space and time called gravitational waves.
Over time, the velocity difference increases, causing the neutron star’s interior to emit superfluid vortices, transferring angular momentum, accelerating the normal component and increasing the rotation rate that we see as pulsar perturbations.
The team suggests that superfluidity in neutron stars is divided into two types, which explains how these vortices form. S-wave superfluidity, which dominates the outer core of the neutron star, provides a relatively tame environment that supports the formation of vortices with whole-number, or “integer,” spins. However, in the inner core of a neutron star, the team thinks p-wave superfluidity dominates, creating extreme conditions that favor half-integer spin vortices.
This means that a whole spin vortex would split into two half-integer vortices upon entering the p-wave dominated inner core. This creates a superfluid structure called a “boojum” that is shaped like a cactus. As more half-vortices are created and connected via boojums, the dynamics of vortex clusters become increasingly complex. Imagine this as the arms of a cactus intertwining with the arms of a neighboring plant, creating increasingly intricate and twisted patterns.
The team ran simulations that showed that their model comes very close to the glitch energies of real neutron star glitches.
“Our argument, while 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,” said team member and associate professor at Nishogakusha University Shigehiro Yasui. “By translating this into a corresponding power-law distribution for glitch energies, we found that it matches the observations.”
“A neutron star is a very special situation because the three fields of astrophysics, nuclear physics and condensed matter physics meet at one point,” Yasui concluded. “It is very difficult to observe directly because neutron stars exist far away from us. Therefore, we need to make a deep connection between the internal structure and some observational data of the neutron star.”
The team’s research is published in the journal Scientific Reports.