A Milky Way magnetar called SGR 1935+2154 may have just massively contributed to solving the mystery of powerful deep-space radio signals that have vexed astronomers for years.
On 28 April 2020, the dead star - sitting just 30,000 light-years away - was recorded by radio observatories around the world, seemingly flaring with a single, millisecond-long burst of incredibly bright radio waves that would have been detectable from another galaxy.
In addition, global and space X-ray observatories recorded a very bright X-ray counterpart.
Work on this event is very preliminary, with astronomers madly scrambling to analyse the swathes of data. But many seem in agreement that it could finally point to the source of fast radio bursts (FRBs).
"This sort of, in most people's minds, settles the origin of FRBs as coming from magnetars," astronomer Shrinivas Kulkarni of Caltech, and member of one of the teams, the STARE2 survey that also detected the radio signal, told ScienceAlert.
Fast radio bursts are one of the most fascinating mysteries in the cosmos. They are extremely powerful radio signals from deep space, galaxies millions of light-years away, some discharging more energy than 500 million Suns. Yet they last less than the blink of an eye - mere milliseconds in duration - and most of them don't repeat, making them very hard to predict, trace, and therefore understand.
Potential explanations have ranged from supernovae to aliens (which, sorry, is extremely unlikely). But one possibility that has been picking up steam is that FRBs are produced by magnetars.
These are a particularly odd type of neutron star, the extremely dense core remnants left over after a massive star goes supernova. But magnetars have much more powerful magnetic fields than ordinary neutron stars - around 1,000 times stronger. How they got that way is something we don't understand well, but it has an interesting effect on the star itself.
As gravitational force tries to keep the star together - an inward force - the magnetic field is so powerful, it distorts the star's shape. This leads to an ongoing tension between the two forces, Kulkarni explained, which occasionally produces gargantuan starquakes and giant magnetar flares.
On 27 April 2020, SGR 1935+2154 was detected and observed by multiple instruments undergoing a spurt of activity, including the Swift Burst Alert Telescope, the AGILE satellite and the NICER ISS payload. It initially looked relatively normal, consistent with behaviour observed in other magnetars.
But then, on April 28, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) - a telescope designed to scan the skies for transient events - made an unprecedented detection, a signal so powerful the system couldn't quite quantify it. The detection was reported on The Astronomer's Telegram.
But the STARE2 survey, a project started by Caltech graduate student Christopher Bochenek, is designed exactly for the detection of local FRBs. It consists of three dipole radio antennas located hundreds of kilometres apart, which firstly can rule out local signals produced by human activities, and can also allow for signal triangulation.
It received the signal loud and clear, with a fluence of over a million jansky milliseconds. Typically, we receive extragalactic FRBs at a few tens of jansky milliseconds. Once corrected for distance, the SGR 1935+2154 would be on the low end of FRB power - but it fits the profile, Kulkarni said.
"If the same signal came from a nearby galaxy, like one of the nearby typical FRB galaxies, it would look like an FRB to us," he told ScienceAlert. "Something like this has never been seen before."
Transient phase space plot now with the SGR 1935+2154 lower limit from STARE2. I think the interpretation writes itself. pic.twitter.com/8ScrlcyqLW— An Dr. Evan Ó Catháin🅾️ (@evanocathain) April 29, 2020
But we also saw something else we've never seen in an extragalactic FRB, and that's the X-ray counterpart. These are quite common in magnetar outbursts, of course. In fact, it is far more normal for magnetars to emit X-ray and gamma radiation than radio waves.
The X-ray counterpart to the SGR 1935+2154 burst was not particularly strong or unusual, said astrophysicist Sandro Mereghetti of the National Institute for Astrophysics in Italy, and research scientist with the ESA's INTEGRAL satellite. But it could imply that there's a lot more to FRBs than we can currently detect.
"This is a very intriguing result and supports the association between FRBs and magnetars," Mereghetti told ScienceAlert.
"The FRB identified up to now are extragalactic. They have never been detected at X/gamma rays. An X-ray burst with luminosity like that of SGR1935 would be undetectable for an extragalactic source."
But that radio signal was undeniable. And, according to Kulkarni, it's absolutely possible for a magnetar to produce even larger outbursts. SGR 1935+2154's burst did not require much energy, for a magnetar, and the star could easily handle a burst a thousand times stronger.
It's certainly giddying stuff. But it's important to bear in mind that this is early days yet. Astronomers are still conducting follow-up observations of the star using some of the most powerful tools we have.
And they have yet to analyse the spectrum of the burst, to determine if it bears any similarities to the spectra of extragalactic fast radio bursts. If it doesn't, we may be back to square one.
Of course, even if SGR 1935+2154 does turn out to confirm a magnetar origin for fast radio bursts, that won't mean it's the only origin. Some of the signals behave very differently, repeating unpredictably. One source was recently found to be repeating on a 16-day cycle.
Whatever SGR 1935+2154 tells us, we are far from completely resolving the complicated enigma these incredible signals represent - but it's an incredibly exciting step forward.
CHIME have not yet responded to ScienceAlert's request for an interview. They were probably a bit too busy.
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