Rapidly spinning dead star slows down, and scientists think they know why

A magnetar — or highly magnetic dead star — with the unassuming name of SGR 1935+2154 is attracting a lot of attention.

Back in October 2020 it suddenly slowed down. A few days after that, there were three fast radio bursts (FBR), and pulsed radio waves for a month coming from the same area.

A new study has looked into what could have caused this, and the team has come to the conclusion that a volcano-like rupture on the dead star’s surface could have caused the slow down or what’s known as a ‘spin-down glitch’.

“People have speculated that neutron stars could have the equivalent of volcanoes on their surface,” said Rice University astrophysicist Professor Matthew Baring.

“Our findings suggest that could be the case and that on this occasion, the rupture was most likely at or near the star’s magnetic pole.”

Magnetars are a type of neutron star — the compact remains of a dead star that collapsed under intense gravity. They can be as dense as the nucleus of an atom, and only a few kilometres wide. They rotate once every few seconds and feature the most intense magnetic fields in the universe.

SGR 1935+2154 has been in the news before. Not long after it glitched, researchers linked the FRBs to SGR 1935+2154, making it the first FRBs to have come from a known object. More excitingly, it was the first FRB inside the Milky Way.

But this doesn’t explain what happened to the magnetar to have caused the spin-down glitch, and both the FRBs and the pulsing radio waves.

Although glitches occur pretty regularly, mostly the magnetars speed up, not slow down. Only one other magnetar has ever been conclusively caught undergoing a spin-down glitch (also known as an anti-glitch).

“In most glitches, the pulsation period gets shorter, meaning the star spins a bit faster than it had been,” said Baring.

“The textbook explanation is that over time, the outer, magnetised layers of the star slow down, but the inner, non-magnetised core does not. This leads to a build-up of stress at the boundary between these two regions, and a glitch signals a sudden transfer of rotational energy from the faster spinning core to the slower spinning crust.”

But spin-down glitches can’t work like that. Instead, the team suggested that a volcano like explosion on the surface could do the trick.

The team suggest that the spin-down glitch occurred when surface plasma close to the magnetic pole of SGR 1935+2154 shed itself. This could have spewed a wind of particles into space, which could alter the star’s magnetic field and become radio emissions.

“Here we unveil the detection of a large spin-down glitch event from the magnetar SGR 1935+2154 on 5 October 2020,” the team write in the new paper.

“Given the rarity of spin-down glitches and radio signals from magnetars, their approximate synchronicity suggests an association, providing pivotal clues to their origin and triggering mechanisms with ramifications to the broader magnetar and FRB populations.”

Considering what we’ve already discovered, this isn’t likely to be the last time we’ll hear of super star magnetar SGR 1935+2154. The research has been published in Nature Astronomy.