It was a dazzling death. Roughly 1.3 billion years ago a star exploded with such force that it was 50 times brighter than the hundred billion stars in its host galaxy combined. It was so bright that if it took place in the Andromeda Galaxy, it would be visible to the naked eye. The outburst, officially known as PTF10hgi, belongs to a rare class of explosions called “superluminous” supernovae, which can shine a hundred times brighter than typical ones. But astronomers cannot say why.
One hypothesis suggests they are powered by magnetars—ultradense, rapidly spinning and highly magnetized cinders of stellar cores that can form in the aftermath of supernova explosions. If those magnetars are spinning fast enough, say 1,000 times a second, they can slow down rapidly by releasing a magnetized wind. That wind, created the moment the magnetar forms, then shocks the ejecta, adding a steadily increasing amount of heat and light to the explosion over the course of several weeks, making it much more luminous than it would be otherwise. But this scenario is only a hypothesis. “The holy grail—the thing that we’re missing—is this direct observational confirmation that there is a magnetar, this beast, in the center of the explosion,” says Brian Metzger, an astronomer at Columbia University. Now a study posted to the preprint server arXiv in January just might provide that holy grail.
Tarraneh Eftekhari, a graduate student at Harvard–Smithsonian Center for Astrophysics, her advisor Edo Berger, Metzger and their colleagues have detected radio light at the precise location where PTF10hgi once erupted. It is the first time astronomers have spotted radio emission in the aftermath of one of these superluminous supernovae. Because radio light is produced when electrons are accelerated within a magnetic field, the finding suggests a magnetar sits squarely in the spot where the supernova burst—potentially solving a near-decade-old mystery. “It’s the first time that we’re peering through the explosion and seeing the engine—seeing the wizard behind the curtain,” Berger says. “That just by itself is quite remarkable.”
Metzger is a little more conservative in his enthusiasm. “This is an exciting hint that we may have the first direct evidence that superluminous supernovae are powered by central magnetars, but we need more observations,” he says. Already, the team has submitted several proposals to make follow-up studies of the object so they can say with certainty a magnetar, and not another culprit, produces the radio emission. And Deanne Coppejans, an astronomer at Northwestern University who was not involved in the study, agrees the future data are crucial. “At the moment it’s looking very promising, but the observations they’ve proposed should solve the mystery,” she says.
Two Sides of the Same Coin
The study might also solve a second mystery—that of fast radio bursts, or FRBs. These brief, bright flashes of radio waves appear to originate in distant galaxies and yet their precise sources remain unknown, making them one of the most intriguing puzzles in astrophysics. Although they might seem unrelated to superluminous supernovae, when Eftekhari and her colleagues picked up the radio emission coincident with PTF10hgi, it was reminiscent of radio light associated with one such FRB. And it was not a surprise to the team—that was precisely the signature Eftekhari and her colleagues had hoped to find.
In 2016 astronomers announced a major clue in the FRB riddle: One of the bursts, known as FRB 121102, flared up more than once, making it the first burst to repeat. The finding allowed scientists to place it on the cosmic map—pinning it to a galaxy roughly 2.5 billion light-years away. Surprisingly, that galaxy was not run-of-the-mill, but rather a dwarf galaxy with few heavy elements—an oddity that looked remarkably similar to the galaxies where superluminous supernovae originate. This caused astronomers to wonder whether the two might somehow be related. In addition, when researchers localized the repeating FRB, they found a persistent source of weaker emission that emanated from the exact spot where the bursts had occurred. That radio emission suggested the burst originated within an intense magnetic field, which could have been produced by a magnetar.
Those two hints led Metzger and Berger to postulate superluminous supernovae and FRBs are two different signatures of the same object. In 2017 they published a study with a number of theoretical calculations that suggested a magnetar would first produce a superluminous supernova—and then, years if not decades later, produce a number of FRBs (although exactly how remains a mystery). And all the while that magnetar would create a nebula that glows in radio light. If the hypothesis is correct, astronomers should be able to look at these superluminous supernovae years after they first erupt and see the nebula’s persistent radio light and—if they are really lucky—a fast radio burst or two.
Tracking More Clues
Recently a number of teams began the hunt for that exact signature. Eftekhari and her colleagues analyzed 25 superluminous supernovae with the Very Large Array (VLA) in New Mexico and the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes. When they spotted this radio source it seemed to confirm their highest hopes, offering not only proof (or close to it) that the superluminous supernova was spawned by a magnetar but a hint the same magnetar might also give rise to FRBs. “It was a little too good to be true that in our first search that we’ve ever done we had such a beautiful result,” Berger says.
And others throughout the field are equally excited. “It’s a tantalizing discovery that really does hint that some of these mysterious objects that have puzzled us for a long time—superluminous supernovae and fast radio bursts—are all manifestations of the same thing,” says Andrew Levan, an astronomer at the University of Warwick in England who did not take part in the study. “It’s a great discovery.”
But it is still just one clue. To transform that into hard evidence, astronomers would like to see an FRB—and not just radio emission—emanate from a past superluminous supernova. “That would be the real smoking-gun connection,” says Casey Law, an astronomer at the University of California, Berkeley, who was not part of the work. And such a find might just be around the corner. A number of scientists are eager to follow up on this object and other superluminous supernovae.
Laura Spitler, an astronomer at Max Planck Institute for Radio Astronomy who discovered the first repeating FRB but was not involved in this research, is planning to observe this object soon. And Law, who has already completed a search for FRBs within superluminous supernovae sites but has yet to analyze the observations, just might have one already hiding within his data.