One of the most interesting stars in the Milky Way is still serving up more than its fair share of intrigue.
In October 2020, SGR 1935+2154, the magnetar responsible for spitting out radio signals never before detected in our home galaxy, unexpectedly slowed down.
Now, scientists believe the rotational slowdown could be evidence of a volcano-like eruption on its surface, spewing material out into space that altered the star’s environment enough to decelerate the spinning of the planet minutely.
It’s a finding that could shed some light on the mystery of fast radio bursts – how these ultra-dense dead stars can spit powerful staccato radio flares across millions of light-years.
“People have speculated that neutron stars could have the equivalent of volcanoes on their surface,” says astrophysicist Matthew Baring of Rice University in Houston, Texas.
“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.”
SGR 1935+2154 burst onto the scene of global fame – quite literally – in May 2020, when astronomers detected it emitting a brief, but powerful, radio flare.
The reason this was exciting was because we’d only ever previously detected such flares from other galaxies. These flares, occurring in radio wavelengths, are just milliseconds in length, emitting up to as much energy in that timeframe as 500 million Suns. And most of them flared once, unexpectedly, and have not been detected since.
Their distance and unpredictability make these fast radio bursts very difficult to learn more about. Astronomers have been able to trace some to the galaxies that emitted them, but figuring down the mechanism or mechanisms behind them was a lot harder to pin down.
SGR 1935+2154 was a breakthrough: here, finally, we could trace a fast radio burst to a specific object.
SGR 1935+2154 is a type of neutron star known as a magnetar.
Neutron stars are already extreme: the ultra-dense cores of massive stars that have gone supernova, blasting off their outer material while the remaining heart of the star collapses under gravity to a sphere packing the mass of up to around 2.4 Suns into a diameter of around 20 kilometers (12 miles).
Add an insanely powerful magnetic field, around 1,000 times more powerful than a normal neutron star’s and a quadrillion times more powerful than Earth’s, and you have a magnetar.
Astronomers speculated that the outward pull of that magnetic field against the inward pressure of gravity could cause the magnetar to occasionally rupture, producing flares and fast radio bursts.
But more information was needed, so SGR 1935+2154 remained under close surveillance. Then, in October 2020, it was caught emitting millisecond radio signals again.
And now, a research team led by astrophysicist George Younes of George Washington University have found that just a few days prior to that activity, it did something really weird: it suddenly slowed down.
Neutron stars have, occasionally, been caught suddenly changing their rotation speed. It’s called a glitch, and it’s a poorly understood phenomenon.
A neutron star glitch is usually a sudden acceleration in the rotation speed. A slowdown, sometimes known as an anti-glitch, is much rarer.
Just three anti-glitches, including SGR 1935+2154, have been detected. And, while a glitch can be explained by changes inside the star, an anti-glitch cannot.
So, the researchers decided to investigate what could have caused it – and what role, if any, the anti-glitch could have played in generating the radio burst activity detected a few days later.
If internal changes could not be the cause of the slowdown, the researchers turned to external explanations.
They constructed a model based on a volcano-like rupture on the magnetar’s surface, ejecting a wind of particles out into the space around the star, postulating that the rarity of both events – the anti-glitch and the radio activity – means that their temporal proximity implies a relationship.
“What makes the October 2020 event unique is that there was a fast radio burst from the magnetar just a few days after the anti-glitch, as well as a switch-on of pulsed, ephemeral radio emission shortly thereafter,” Baring says.
“We’ve seen only a handful of transient pulsed radio magnetars, and this is the first time we’ve seen a radio switch-on of a magnetar almost contemporaneous with an anti-glitch.”
And, according to their model, a rupture close to the stellar pole could have generated a wind that interacts with the magnetar’s magnetic field, slowing down the star’s rotation rate, and changing the geometry of the magnetic field in a way that could enhance the conditions for radio emission.
A powerful, massive wind blowing for just a few hours from a volcano-like spot could create the conditions needed for the slow-down and the subsequent radio activity, the team found.
“The wind interpretation provides a path to understanding why the radio emission switches on,” Baring says.
“It provides new insight we have not had before.”
The research has been published in Nature Astronomy.