Astrophysicists had thought that if a magnetar ever exploded, it would release one of the highest bursts of energy ever seen in the universe. But until now they could never prove it. Then one of these unusual neutron stars flashed in a nearby galaxy. The flare of energy it released was truly enormous!
Magnetars are neutron stars— stellar corpses — possessing the most extreme magnetic fields known. Those fields are so intense that they will heat the magnetar’s surface to 10 million degrees Celsius (18 million degrees Fahrenheit).
The first sign of the newfound magnetar arrived as a blast of X-rays and gamma rays. Five telescopes in space observed the flare on April 15, 2020. Among them were the Fermi Gamma-ray Space Telescope and the Mars Odyssey orbiter. Together, these eyes in the sky offered enough information to track down the flare’s source. It was the Sculptor galaxy, 11.4 million light-years away.
Astronomers had seen flaring magnetars in the Milky Way. But they were so bright that it was impossible to get a good enough look at them and measure their brightness. Possible glimpses of flaring magnetars in other galaxies may have been spotted before, too. But “the others were all a little circumstantial,” says Victoria Kaspi. They were “not as rock solid” as the newfound one, she says. Kaspi is astrophysicist at the McGill Space Institute in Montreal, Canada. She was not involved in the new discovery. “Here you have something that is so incontrovertible,” she says. “It’s like, okay, this is it. There’s no question anymore.”
Astronomers reported the find January 13 at the virtual meeting of the American Astronomical Society. Additional details were described in papers the same day in Nature and Nature Astronomy. It’s the first time astronomers had identified an exploding magnetar in another galaxy.
How the magnetar was ID’d
When astronomers saw the cataclysmic explosion, they at first thought it was something called a short gamma-ray burst, or GRB. Most such flares develop when two neutron stars collide or there is some other destructive cosmic event.
But the signal looked weird. Its brightness peaked quickly — in just two milliseconds. The light then tailed off for another 50 milliseconds. Within about 140 milliseconds, the whole light show appeared to be over. As the signal faded, some telescopes also detected fluctuations in the light. Those changes occurred on timescales faster than a millisecond.
Typical short GRBs from a neutron-star collision don’t change like that, notes Oliver Roberts. He’s an astrophysicist at the Universities Space Research Association. It’s in Huntsville, Ala. But flaring magnetars in our own galaxy do show such light dynamics. The bright flare comes in and out of view as the magnetar spins.
Another odd trait of the new flare: Four minutes after the initial blast, the Fermi telescope caught incoming gamma rays. They had energies higher than a giga-electronvolt. No known source of GRBs spew those.
As a result, concludes Kevin Hurley, “We’ve discovered a masquerading magnetar in a nearby galaxy. And we’ve unmasked it,” adds this astrophysicist of the University of California, Berkeley. He spoke at a January 13 news briefing.
The researchers think that the flare was triggered by a massive starquake. A truly big one. It would appear to have been 1,000 trillion trillion (or 1027) times as large as the 9.5 magnitude earthquake recorded in Chile in 1960. “I’m from California,” says Hurley. “And out here we would definitely call that the Big One,” he says.
This cosmic quake led the magnetar to release a blob of plasma. It sped away at nearly the speed of light. Along its path, it emitted X-rays and even higher energy gamma rays.
Were some earlier magnetar flares misdiagnosed?
The new find suggests that at least some signals that look like short GRBs are in fact magnetar flares. Astronomers had long suspected this. In fact, the new flare suggests that three earlier events that astronomers had flagged as possible magnetar flares likely were just that. Astronomers now have at least four magnetar flares to compare with each other.
The latest flare also could have exciting implications for another type of mystery signal from deep space. Called fast radio bursts, these flares have left astronomers scratching their heads for more than a decade. Several types of evidence connect fast radio bursts to magnetars. These include another signal coming from within the Milky Way. It also arrived in April 2020.
Kaspi at the McGill Space Institute has compared the apparent frequency of magnetar flares in other galaxies to the frequency of fast radio bursts. And, she finds, their rates are similar. “That argues that actually, most or all fast radio bursts could be magnetars.”