Magnetars certainly know how to make an entrance. A recent study suggests that these highly magnetized stars make their cosmic debut amid the brightest flares of radiation in the universe, called ultralong-duration gamma-ray bursts (GRBs). This discovery ties together some of the most magnetic and energetic phenomena in the cosmos and sheds light on the mysterious origins of ultralong-duration GRBs.
GRBs are blasts of gamma-ray radiation that typically fade after a few seconds, but on rare occasions can last up to a half hour. The majority of these events are “long-duration” GRBs. Whereas normal GRBs are likely formed by the merging of two neutron stars, scientists think that long GRBs are forged in the explosive deaths of massive stars called supernovae, says lead author Jochen Greiner, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Germany.
When a massive star explodes, part of its material is ejected into space whereas the rest collapses into a remnant neutron star or black hole. This violent death spawns two jets of material that spew from opposite sides of the remnant at nearly light-speed. An observer looking down the barrel of one of these jets sees a GRB. The light we recognize as the supernova blooms out from the exploded star in all directions and takes longer to rise, explains Andrew Levan, an astrophysicist at the University of Warwick in England who was not involved with this study. As the GRB “afterglow” fades, the supernova becomes visible, producing a small “bump” in the measured radiation levels.
Scientists pinned down the origin of long GRBs over a decade ago, but in recent years a new class of ultralong GRBs has cropped up, with no apparent link to supernovae. Only four of these bursts have been observed—as compared with the few thousand known “normal” long GRBs—and they shine for several hours. “We've been puzzling over these ultralong bursts for awhile now,” Levan says. Some thought exceedingly massive collapsing stars (aptly called “collapsars”) were responsible whereas others proposed that ultralong GRBs were powered by black holes shredding stars.
Now, for the first time, Greiner and his colleagues have established a clear link between the ultralong GRB 111209A and a supernova, SN 2011kl. But this is not just any stellar explosion. Greiner’s team thinks that the supernova SN 2011kl created a magnetar—a tiny neutron star spinning hundreds of times every second with a magnetic field a quadrillion times stronger than Earth’s. Magnetars are the most magnetic objects in the known universe.
Scientists were first alerted to the flare of GRB 111209A by the Swift Gamma-Ray Burst satellite on December 9, 2011. The event persisted for a few hours, all under the vigilant watch of the Konus gamma-ray detector onboard the interplanetary WIND spacecraft. For the 70 days that followed scientists tracked 111209A’s afterglow. But it wasn’t until last June that Greiner’s team realized this burst was the key to understanding the origins of ultralong GRBs. Supernova data collected by another group of scientists using the X-Shooter spectrometer on the Very Large Telescope in Chile had recently become public, showing there was in fact a supernova associated with GRB 111209A.
The supernova, SN 2011kl, was more than three times as bright as the supernovae previously associated with long GRBs, and its spectrum was completely different. SN 2011kl's characteristics required the supernova to get an extra boost of energy from somewhere. Greiner’s team found that only a magnetar, whose rapid rotation and monster magnetic fields constitute a huge reservoir of energy, could create such a bright flare. The scientists suspect that the magnetar forged in the stellar explosion reenergized the material billowing out from the star in all directions, making for an extremely bright supernova. The magnetar’s immense power similarly and generated the extremely long-lasting jets seen as GRB 111209A. Given the supernova’s distance and the speed of light, scientists estimate this magnetar was born about 6.5 billion years ago.
According to Levan, the link between GRB 111209A and SN 2011kl is crucial not only for uncovering the origins of ultralong GRBs, but also understanding how the supernova itself burned so brightly, because SN 2011kl looks similar to a recently discovered class of superluminous supernovae whose energy source still remains a mystery. “So this new result provides something of a unifying view of a diverse range of stellar collapses,” Levan says. “If correct, it means that magnetars might be the driving force between many of the most energetic events in nature.” At the very least, this new research shows that 111209A and its ultralong fellows fit in with the long GRB family after all.