A dead star 14,350 light-years away has just become the most important clue in solving the mystery of fast radio bursts. Earlier this year, it spat out a colossal, milliseconds-long radio flare – and now the first published analysis of the event notes its similarity to the enigmatic extragalactic signals.
Fast radio bursts (FRBs) are a mystery that has perplexed astronomers ever since the first one was discovered in 2007. They are bursts of extremely powerful radio waves from galaxies millions of light-years away, some discharging more energy than hundreds of millions of Suns. And they last just milliseconds.
Because most of the FRBs detected to date are one-off, non-repeating events, that come from very far away, and can’t be predicted, they’ve proven extremely challenging to track down, and therefore figure out. Proposed explanations have ranged from supernovae to aliens (extremely unlikely), but one candidate has shown increasing promise: magnetars.
In the case of the event earlier this year, it was a magnetar called SGR 1935+2154 that was detected emitting a millisecond-duration burst of radio waves by instruments around the world.
“This is the first ever observational connection between magnetars and Fast Radio Bursts,” said astrophysicist Sandro Mereghetti of the National Institute for Astrophysics in Italy.
“It truly is a major discovery, and helps to bring the origin of these mysterious phenomena into focus.”
Magnetars are a type of neutron star – the dead remnant of a massive star after it has blown off most of its mass in a supernova – with extremely powerful magnetic fields, 1,000 times more powerful than normal neutron stars
These powerful magnetic fields have a strange effect. As gravity applies an inward force keeping the star together, the magnetic field pulls outward, distorting the star’s shape.
These two ongoing, competing forces create a tension that occasionally results in massive starquakes. These are called magnetar outbursts, and they usually produce X-rays and gamma rays. Only very rarely have magnetars been caught emitting radio waves.
Astronomers pay attention to magnetar outbursts because we don’t know a lot about how their magnetic fields are the way they are, and any activity we can observe of the phenomenon could help shed some light. So when SGR 1935+2154 started getting rumbly in late April, monitoring instruments around the world were turned in its direction.
Initially, it looked like a pretty standard magnetar outburst, but on 28 April, the unprecedented occurred: a very bright radio flare that looked shockingly similar to a fast radio burst, detected by multiple instruments.
It was so bright that the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope – designed to detect transient events, and responsible for discovering a good number of FRBs – couldn’t quite quantify it.
That’s not because the flare was intrinsically more powerful than extragalactic FRBs (it was actually intrinsically weaker), but because it was so much closer.
By using data collected by the European Space Agency’s INTEGRAL satellite, Mereghetti and his team positively associated the signal with the magnetar, and analysed and characterised it.
“Crucially, the IBIS imager on Integral allowed us to precisely pinpoint the origin of the burst, nailing its association with the magnetar,” said astrophysicist Volodymyr Savchenko of the University of Geneva in Switzerland.
“Most of the other satellites involved in the collaborative study of this event weren’t able to measure its position in the sky – and this was crucial in identifying that the emission did indeed come from SGR 1935+2154.”
Although the flare itself was quite a bit weaker than extragalactic FRBs, almost everything else about it fits the extragalactic FRB profile. But there was a surprise, too – the radio burst had an X-ray counterpart, something we’ve never seen in an extragalactic FRB.
That doesn’t mean that extragalactic FRBs don’t have X-ray counterparts; in fact, it could mean the opposite, that the signals are more complex than we thought, spewing out multiple types of radiation below our detection threshold.
“This is a very intriguing result and supports the association between FRBs and magnetars,” Mereghetti told ScienceAlert earlier this year.
“The FRBs identified up to now are extragalactic. They have never been detected at X/gamma rays. An X-ray burst with luminosity like that of SGR 1935+2154 would be undetectable for an extragalactic source.”
In this case, the X-ray counterpart allowed the team to refine distance measurements to the magnetar. Previously, it was thought to be around 30,000 light-years away.
Although this is extremely convincing evidence in favour of the magnetar origin for FRBs, it would be a mistake to call the mystery conclusively solved. It’s possible that there are other sources, especially because some of the signals behave very differently.
Some are stronger, some weaker. Some repeat. Most don’t. Two have even been caught repeating on a cycle.
So this probably won’t be the last we hear from SGR 1935+2154. It’s the first detection of its kind, and astronomers around the world are tremendously excited. It’s well on its way to becoming one of the most studied magnetars in the Milky Way – and this is just the beginning.
The research has been published in The Astrophysical Journal Letters.
