Artist's impression of a pulsar. (Pitris/Getty Images) SPACE Unexplained Glitches in Pulsars Could Be Evidence of Elusive 'Strange Nugget' Particles MICHELLE STARR 17 JULY 2020
Every now and again, some dead stars called pulsars do something quite odd. They sort of hiccup, making an abrupt acceleration in their rotation speed before – usually – gradually slowing to normal.
We're not sure what causes them; astronomers believe it is some sort of internal process, such as starquakes, or the unpinning of vortices in the star's superfluid core. But some glitches are even weirder. They are much smaller than the classical glitches, and their signatures are much more challenging to explain.
A new paper led by physicist Innocent Okwudili Eya of the University of Nigeria proposes a slightly more daring explanation. He and his team make the case that the cause of these tiny glitches could be hypothetical particles called… strange nuggets.
"In this analysis, we suggest that accretion of strange nuggets with pulsars could readily give rise to microglitch events," they wrote in their paper.
"The signature of the events depends on the energy of the strange nuggets and line of interaction."
Pulsars are pretty interesting stars. They're a type of neutron star – the cores of dead stars of a certain mass after they have gone supernova.
But pulsars are highly magnetised, spin very, very fast, often on millisecond scales, and they're oriented in such a way that beams of radiation from their poles appear to pulse as they rotate past, like a lighthouse.
Some of them pulse with incredible regularity; that makes them useful for such applications as calculating distances, understanding the interstellar medium, and probing the curved spacetime around a black hole.
We could even potentially use them like a GPS to navigate the stars.
Approximately 190 of the 2,700 known pulsars have been recorded glitching; between them, there are more than 500 classical glitches on record. Generally, the pulsar slows fractionally over time. The glitch is like a hiccup that corrects the rotational velocity of the pulsar.
But, while we're not sure what causes them, we understand even less about microglitches.
These can be orders of magnitudes smaller than conventional glitches, and some of them have a negative signature – that is, a deceleration, also known as an anti-glitch – that, the researchers assert, is inconsistent with conventional glitch mechanisms.
This is where the strange nuggets – AKA strangelets – enter the picture. These are a hypothetical particle that we've never actually detected, and that some scientists have put forward as a candidate for dark matter.
They're made up of quarks. That's not unusual; protons and neutrons – the subatomic particles in the atomic nuclei that make up all visible matter (including us) – each contain three quarks. It's the flavours of quarks that make the difference.
Quarks come in six 'flavours' – up, down, top, bottom, strange, and charm. Two up quarks and one down quark make a proton; two down quarks and one up quark make a neutron.
The other four flavours are 'heavy' quarks. Thanks to their low stability, they tend to only be seen in particle colliders, where they decay rapidly into other particles soon after forming.
But it's hypothesised that strange quarks could be stabilised if combined in equal numbers with up and down quarks. Such a particle is called a strangelet, strange quark, or strange nugget – a small piece of strange quark matter (also hypothetical).
It's possible that strangelets were created along with protons and neutrons in the early Universe, when the primordial quark soup coalesced into matter. If so, they could still be hanging around the cosmos today.
In 2009, a different team of astronomers argued that pulsars made entirely of strange quark matter could be responsible for microglitches; but Eya and his team have a different explanation.
They studied 299 recorded microglitches, and, according to their calculations, the events could be caused by strange nuggets colliding with the pulsars.
If strange nuggets are indeed still hanging around, they could collide with stellar objects. Pulsars would be good candidates for finding such collisions – they have a strong gravitational field, and their regular pulses would make the collisions more detectable than, say, non-pulsar neutron stars.
And variations in the energy of the colliding strange nuggets could explain both the positive and negative microglitches.
"In these frameworks, the interaction of pulsars with SNs could lead to starquake (or cracking of the crust) if the impact is high enough," the researchers wrote in their paper.
"The quake readily lead to sudden spin-ups observed as microglitches of positive signatures. In contrast, if the energy of the strange nuggets is not large enough for the impact to cause a starquake, the interaction leads to sudden increase in stellar moment of inertia, which manifests in sudden spin-down observed as microglitches of negative signatures."
It is, of course, theoretical, and some might argue unlikely. But it's hard to make progress in science if you aren't at least open to exploring new possibilities and having fun. And it's hard to test a hypothesis if you don't have a hypothesis to test.
So we'll be eagerly awaiting for the next development in this line of inquiry.
The research has been published in Astrophysics and Space Science.