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Magnetars drag spacetime to power superluminous supernovae - Ars Technica
Frame-dragging may explain an odd pattern seen in the brightest supernovae.
Some of the most extreme explosions in the universe are Type I superluminous supernovae. “They are one of the brightest explosions in the Universe,” says Joseph Farah, an astrophysicist at the University of California, Santa Barbara. For years, astrophysicists tried to understand what exactly makes superluminous supernovae so absurdly powerful. Now it seems like we may finally have some answers.
Farah and his colleagues have found that these events are most likely powered by magnetars, rapidly spinning neutron stars that warp the very space and time around them.
Magnetars have been a leading candidate for the engine behind superluminous supernovae. The theory says these insanely magnetized stars are born from the collapsing core of the original progenitor star and emit energy via magnetic dipole radiation. “This core is roughly a one solar mass object that gets crushed down to the size of a city,” Farah explains. As its spin slows down, a magnetar bleeds its rotational energy into the expanding material of the dead star, lighting it up.
The problem was that this theory did not quite explain observations. In a standard magnetar model, the light curve of the supernova should rise rapidly and then fade away evenly as the neutron star loses its rotational energy. “This way the light curve, in the prediction of this model, just goes up and then down quite smoothly,” Farah says. But when astronomers observe superluminous supernovae, they almost never see this smooth fade. Instead, they see bumps, wiggles, and strange modulations. The light curve flickers over months.
For a while, scientists tried to patch the magnetar engine theory to fit observations. Maybe the expanding debris was slamming into irregular shells of material shed by the star before it died. Or perhaps the magnetar engine was spitting out random, violent flares. But these explanations required highly specific, fine-tuned parameters to match what we were seeing through our telescopes.
The solution to the strange flickering problem came when the Liverpool Gravitational Wave Optical Transient Observer collaboration detected an object designated SN 2024afav on December 12, 2024. Initially, the object looked like a standard superluminous supernova. “It was as bright and it had bumps in the light curve like many other objects of this kind,” Farah says. But as the telescopes kept watching, it started doing something unprecedented: It started to chirp.
In physics, a chirp refers to a signal with a frequency that steadily increases over time. In the case of SN 2024afav, its emissions were bumping up and down, but the gap between these bumps was shrinking. After a second and third bump both appeared with the gaps between them reduced by roughly 35 percent, Farah and his team realized they could calculate how much the gap between the bumps would decrease next.
The team adjusted their observation schedule, pointed their instruments at SN 2024afav, and discovered the fourth bump appeared exactly when they expected it would. The fifth bump enabled the scientists to narrow down the period reduction to about 29 percent.
The fact that Farah and his colleagues could accurately predict the bumps delivered a massive blow to our existing magnetar models. While a few irregular bumps could be explained away by the supernova ejecta crashing into clouds of gas, it doesn’t explain perfectly timed, cleanly sinusoidal modulations with a steadily decaying period. Random space rubble just doesn’t work that way.
“So, we came up with the new model to describe this behavior,” Farah explains. They proposed a new physical mechanism that relied on the Lense-Thirring effect, otherwise known as frame-dragging. Frame-dragging is a prediction of General Relativity, where a massive spinning object slightly drags the spacetime around with it as it rotates. “We didn’t try this mechanism before because it had never been seen around a magnetar before,” Farah says. But when his team did try it, it turned out to perfectly match what was going on.
The flickering in the superluminous supernovae, Farah hypothesized, was caused by the extreme gravity of a newborn magnetar dragging the very spacetime around it along as it was spinning.
To understand Farah’s Lense-Thirring solution, imagine a bowling ball spinning in a vat of molasses. As the ball rotates, friction drags the sticky fluid along, creating a swirling vortex. According to Einstein’s General Relativity, mass and energy can warp the fabric of spacetime, so if a sufficiently large mass is spinning rapidly, it drags the space-time along in a manner similar to the molasses. Around Earth, this effect is minuscule. But around a newborn magnetar, which is far more massive and spinning hundreds of times a second, spacetime is whipped into a violent, twisting frenzy.
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