Aussie astroboffins think they have worked out one of the more unusual oddities in the universe – glitchy pulsars.
Neutron stars rotate rapidly, emitting pulses of electromagnetic energy at regular intervals. Sometimes the flashes fluctuate, however, speeding up then slowing down suddenly for several seconds, leaving scientists puzzled.
The odd behavior known as a "glitch" is best observed in the Vela Pulsar, a neutron star located 1,000 light years away. About 5 per cent of pulsars are known to glitch, but Vela glitches once every three years.
A group of scientists – led by a team from Australia's Monash University – pored over data taken from a glitch event in 2016 using the Mount Pleasant Radio Observatory in Tasmania. They discovered that the star's rotational frequency increased by about 16 microhertz, a tiny amount, over 30 seconds or so, according to a paper published in Nature Astronomy on Monday.
That amounts to "about one part in a million," Gregory Ashton, first author of the paper and an assistant astrophysics lecturer at Monash University, told The Register.
"Immediately before the glitch, we noticed that the star seems to slow down its rotation rate before spinning back up. We actually have no idea why this is, and it's the first time it's ever been seen," he added.
The glitch can be explained by visualizing a neutron star as three separate parts. The outer crust is made up of rigid neutrons that stay fixed in place. The inner crust and core, however, behaves more like a superfluid, where neutrons can move and bob around. Glitches are thought to be caused when excess angular momentum from neutrons in the inner crust is transferred to the particles in the outer crust.
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The extra momentum temporarily boosts the rotation rate of neutron stars like the Vesa Pulsar. What triggers this process, however, isn't well understood. After the glitch, the star's rotational frequency tapers off, Paul Lasky, an astrophysics lecturer at Monash University, explained. "A second soup of superfluid that moves in the core catches up to the first causing the spin of the star to slow back down."
What's more puzzling is the drop in rotational frequency immediately before the glitch, something the researchers have called an "anti-glitch".
"To the best of our knowledge this has not been predicted," they concluded in the paper. "We hypothesize that it may be a statistical fluctuation consistent with the overall noise fluctuations and speculate such fluctuations drive the differential lag between the superfluid and the crust above its critical value, thus triggering the glitch."
"We hope to spark a debate about these results in the community," Ashton said. "There are a lot of ideas out there which could explain the observations. By directly testing models on the data itself we hope to quantify how those models stack up against the data." ®