Concept art of a magnetar. Image: Ohlmann/Schneider/Röpke
Imagine this, if you can: two stars spiral toward each other until they merge into one unified star. This star lives on for a while, but eventually explodes in a colossal supernova that transforms it into the strongest type of magnet in the entire universe.Scientists think this bizarre sequence of events might explain the existence of magnetars, which are special types of neutron stars with the most powerful magnetic fields ever detected. This hypothesis not only accounts for the otherworldly properties of magnetars, but may also shed light on other unexplained phenomena, such as fast radio bursts, according to a study published on Wednesday in Nature.
Led by Fabian Schneider, an astronomer at Heidelberg University, the team used advanced supercomputers to show how stellar mergers can “explain the strong magnetic fields observed in a subset of massive stars and potentially also the origin of magnetars,” according to the study.“At the end of the life of the merged star, a supernova explosion will likely produce a magnetar,” Schneider said in an email. “Our paper is about the merging of stars and the formation of strong magnetic fields.”About one in ten stars with a mass more than 1.5 times that of the Sun displays these strong magnetic fields, which fortuitously matches the predicted fraction of massive stars that have undergone mergers, Schneider’s team said in the study.Scientists have been fascinated by magnetars since 1979, when a blast of high-energy gamma rays from one of these exotic objects was first detected. Over the decades, observations and models have revealed that magnetars have magnetic fields that are a million billion times stronger than Earth’s field.Much like “normal” neutron stars, magnetars are created when stars explode and collapse into super-dense spheres that contain the mass of roughly two Suns within a 12-mile diameter. But magnetars end up with about 1,000 times the magnetic strength of regular neutron stars, a distinction that has perplexed scientists for years.Schneider’s team used the AREPO code, a sophisticated cosmological simulation run by supercomputers at the Heidelberg Institute for Theoretical Studies, to model the stellar merger hypothesis of magnetar formation.
The researchers selected Tau Scorpii, a star located about 500 light years from Earth, as an example of a likely magnetar-in-the-making. Tau Scorpii is a strongly magnetic “blue straggler” star, which means it is probably the product of a stellar merger.Schneider and his colleagues ran simulations with AREPO to show that Tau Scorpii’s properties can be explained by a spike in turbulence during a collision of two stars. When Tau Scorpii eventually erupts in a supernova, this strong field affects explosions in the stellar core as it collapses, leading to the formation of a magnetar, the team said.Fast radio bursts (FRBs)—mysterious pulses of radio light that have yet to be explained—may be emitted during this process of magnetar formation. Some FRBs only light up with one pulse, but scientists have also observed FRBs that emit repeated pulses of radio waves.Schneider said that these repeating FRBs might be produced by the shifting magnetic fields of a magnetar, though he added that this is “pure speculation and there are probably more theories on what FRBs are than detected FRBs.”While the origins of FRBs remain a wild card, Schneider’s team concluded that “massive blue straggler stars seem likely to be progenitors of magnetars” and that “their supernovae may be affected by their strong magnetic fields.”It will take more observations and models to corroborate the new research. But the study adds to the evidence that two stars can get a major magnetic boost when they team up as one, and that process may explain the origins of the strongest magnets in the universe.“We have run the very first three-dimensional simulation of two massive stars merging that can follow the magnetic field amplification process,” Schneider said. “This enables us to study stellar mergers now in a broader context—in the end, we think that about 25% of all massive stars will merge with a companion! A whole new world to explore.”Update: This article has been updated to include comments from lead author Fabian Schneider.