Scientists may have cracked the secret of the still-beating hearts of the universe's most extreme "dead stars," and the explanation is twisted.
The team thinks an avalanche of quantum tornados causes this "glitching" in the spin of a class of neutron stars called pulsars when it becomes entangled with its neighbors like the arms of a series of cacti in close proximity, creating twisted and complex patterns.
"More than half a century has passed since the discovery of neutron stars, but the mechanism of why glitches happen is not yet understood," team member and Hiroshima University professor Muneto Nitta said in a statement. "So 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 glitches as the result of a "quantum vortex network" that aligns with power law calculations, thus developing a model that needs no "extra tuning," unlike prior neutron star glitch models.
Neutron star 'glitches' go deep
Neutron stars are born when massive stars die, running out of fuel for nuclear fusion and collapsing under their own gravity. Their outer layers are blown away in huge supernova explosions. This leaves a stellar core with between one and two times the mass of the sun, crushing down to a diameter of around 12 miles (kilometers). That is small enough to fit in the average city on Earth.
The consequence of this collapse is that electrons and protons are crushed together, creating a sea of neutrons that is so dense that if a tablespoon of it were brought to Earth, it would weigh more than 1 billion tons, outweighing Mount Everest.
The crushing down of stellar cores is also responsible for the rapid rotation of young neutron stars, with some reaching speeds of up to 700 rotations per second. This is because of the conservation of angular momentum, which is akin to an ice skater on Earth drawing in their arms to increase the speed of their spin.
Freshly "deceased" neutron stars or "pulsars" appear to pulse because as they rapidly spin they blast beams of radiation from their poles. Pulsars brighten periodically when their beams are pointed directly at Earth, making them appear to pulse (hence their name). This pulsing can be compared 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 hitch, however. Some neutron stars appear to occasionally "glitch," briefly speeding up their rotation and the delivery of their pulses, thus disrupting the regularity of their heartbeat. 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 quakes, low-energy glitches occur more often for pulsars than high-energy and extreme glitches.
There are two prevailing mechanisms related to neutron star glitches: starquakes and tiny quantum vortex "avalanches" that form like microscopic hurricanes in the superfluid soup that composes a neutron star's interior.
Quantum vortices are generally more widely accepted as an explanation than starquakes because, while starquakes would follow a power law like earthquakes, they struggle to account for all types of neutron star glitches. Yet, despite being more widely accepted, there is no real explanation of what could trigger a catastrophic avalanche of superfluid vortices that can reach the surface of a neutron star and cause it to increase its spin speed.
"In the standard scenario, researchers consider that avalanche of unpinned vortices could explain the origin of glitches," Nitta explains in the press release. "If there would be no pinning, it means the superfluid releases vortices one by one, allowing for a smooth adjustment in rotation speed. There would be no avalanches and no glitches."
Nitta added that the team's model does not need an additional pinning mechanism. This model only needs to consider a structure consisting of two types of waves rippling through a neutron star's superfluid interior: a "P wave," which is a fast-moving longitudinal wave, and an "S wave," which is a slower-moving transverse wave.
"In this structure, all vortices are connected to each other in each cluster, so they cannot be released one by one," Nitta continued. "Instead, the neutron star has to release a large number of vortices simultaneously. That is the key point of our model."
Ordinary matter in neutron stars is a drag
The team's model suggests that a neutron star's superfluid core spins at a constant pace, but the non-superfluid "ordinary" component drags on it. The result is the slowing of the neutron star's rotation speed by the emission of electromagnetic pulses and tiny ripples in space and time called gravitational waves.
Over time, the difference in speeds grows, resulting in the neutron star interior expelling superfluid vortices, carrying angular momentum, speeding up the ordinary component, and causing the increase in rotation rates we see as pulsar glitches.
The team suggests that superfluid in neutron stars is divided into two types, which explain how these vortices are born. S-wave superfluids, which dominate the outer core of the neutron star, provide a relatively tame environment that supports the formation of vortices that have whole number, or "integer," spins. However, in the inner core of a neutron star, the team thinks that p-wave superfluidity dominates, creating extreme conditions that favor half-integer spin vortices.
That means a whole integer spin vortex would split into two half-integer vortices when 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 through boojums, the dynamics of vortex clusters become increasingly complex. Imagine this as being like a cacti's arms intertwining with a neighboring plant's arms, creating increasingly intricate and twisted patterns.
The team conducted simulations that showed their model comes very close to replicating the glitch energies of real-world 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," team member and Nishogakusha University associate professor Shigehiro Yasui said. "Translating this into a corresponding power-law distribution for glitch energies showed it matches the observations."
"A neutron star is a very particular situation because the three fields of astrophysics, nuclear physics, and condensed matter physics meet at one point," Yasui concluded. "It's very difficult to observe directly because neutron stars exist far away from us. Therefore, we need to make a deep connection between the interior structure and some observation data from the neutron star."
The team's research is published in the journal Scientific Reports.
