The first object within the Milky Way galaxy caught emitting fast radio bursts is now officially a repeater.
In a new peer-reviewed paper, SGR 1935+2154 has been described spitting out two more powerful radio signals consistent with those seen from extragalactic sources.
The new signals, however, are not all the same strength. This suggests that there could be more than one process inside magnetars that are capable of producing these enigmatic bursts – and that SGR 1935+2154 could be a dream come true, an excellent laboratory for understanding them.
Fast radio bursts have been a puzzle since their discovery in 2007. They’re extremely powerful bursts of energy only in radio frequencies, lasting just milliseconds long at most. And there were several major difficulties in figuring out what they were.
Until April of this year, fast radio bursts (FRBs) had only been detected coming from outside the Milky Way, millions of light-years away – way too far to do more than, at most, track them down to a general region in another galaxy. For most of them, though, we haven’t even been able to do that.
On 28 April 2020, a dead, highly magnetised star within our own galaxy, just 30,000 light-years away, was recorded emitting an incredibly powerful, millisecond duration burst of radio waves.
Once the signal was corrected for distance, astronomers found it was not quite as powerful as extragalactic FRBs, but everything else about it fit the profile. The event was officially confirmed as an FRB earlier this month, and given a name – FRB 200428.
Since then, astronomers have been keeping a careful eye on FRB 200428. And, sure enough, on 24 May 2020, the Westerbork Synthesis Radio Telescope in the Netherlands caught two millisecond-long radio bursts from the magnetar, 1.4 seconds apart.
A much fainter FRB signal was also detected by the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) in China on 3 May.
And already these three new signals are telling us a lot, as described in a paper led by astrophysicist Franz Kirsten of Chalmers University of Technology in Sweden.
The initial April bursts from FRB 200428 were extremely bright – a combined fluence of 700 kilojansky milliseconds. The three follow-up signals were much fainter.
FAST’s was the faintest, at 60 millijansky milliseconds. The two signals from Westerbork were 110 jansky milliseconds and 24 jansky milliseconds respectively.
That’s quite a range of signal strength, and it’s unclear why.
“Assuming that a single emission mechanism is responsible for all reported radio bursts from SGR 1935+2154, it has to be of such a type that the burst rate is close to independent of the amount of energy emitted across more than seven orders of magnitude,” the researchers wrote in their paper.
“Alternatively, different parts of the emission cone might cross our line of sight if the beaming direction changes notably over time.”
Magnetars are funny beasts. They’re a type of neutron star – the tiny collapsed core of a dead star, about 1.1 to 2.5 times the mass of the Sun, but packed into a sphere just 20 kilometres (12 miles) across.
Magnetars add to this 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.
We don’t really know how they form (recent evidence suggests that colliding neutron stars could be one way), but we know they go through periods of intense disruption and activity.
As gravity pushes inward to try to keep the star together, the magnetic field pulls outward, distorting the magnetar’s shape. The two competing forces are thought to produce instabilities, magnetar quakes and magnetar flares, usually seen in high-energy X-rays and gamma radiation.
SGR 1935+2154 is known to go through periods of X-ray activity; that’s fairly normal for a magnetar. But the first FRB – that 28 April one – was also accompanied by an X-ray flare, something that had never been seen before in an FRB. The three new signals, however, showed no signs of X-ray counterparts.
And, when the team worked in the opposite direction, studying X-ray data from the magnetar to try to link it to radio counterparts, they found nothing there, either.
“Therefore it seems that the majority of X-ray/gamma-ray bursts are not associated with pulsed radio emission,” the researchers wrote.
“The parameters and fluences that we measure for the X-ray bursts are consistent with typical values observed for SGR 1935+2154, fitting with the idea that radio bursts are instead associated with atypical, harder-X-ray bursts.
And some questions remain. Some fast radio burst sources exhibit periodicity – a pattern – in their signals.
We haven’t seen that with SGR 1935+2154. It’s possible that we don’t have enough data. It’s possible those periodic FRBs are in binary systems. And it’s eminently possible that magnetars are only one source of FRBs, and others remain to be discovered.
But the magnetar isn’t done yet.
On 8 October 2020, it was recorded spitting out three more radio bursts, in a three-second period. That data is still under analysis, but it marks the beginning of a good collection of signals that could help us look for patterns, or clues as to the magnetar behaviour that spits them out (another recent paper suggests that magnetar quakes are responsible).
“So SGR 1935+2154 is not a flawless analogue of the extragalactic FRB population. Nonetheless, magnetars can plausibly explain the diverse phenomena observed from FRBs,” the researchers wrote in their paper.
“Perhaps the distant, periodically active FRB sources are brighter and more active because they are substantially younger than SGR 1935+2154 and because their magnetospheres are perturbed by the ionised wind of a nearby companion. Similarly, perhaps non-repeating FRBs are older, non-interacting and thus less active. Detailed characterisation of FRB local environments is critical to investigating these possibilities.”
The research has been published in Nature Astronomy.