Researchers led by a graduate student find for the first time that sympathetic flares occur on many stars outside our solar system
Our Sun is a roiling mass of energy, with solar flares exploding on its surface, sending gas, plasma, and light that blasts across the solar system. When radiation from extra-powerful flares breaks through Earth’s outer protective magnetosphere, it can affect satellites and even electric grids and cause the aurora borealis—lighting up the night sky.
Astrophysicists have known for a long time that when our Sun emits flares, some of them trigger a secondary or “sympathetic” flare, which occurs about 5% of the time. It is unclear what mechanism causes the sympathetic flares, and researchers haven’t known if they occurred on other stars, too.
Now scientists at Tufts, led by astronomy doctoral student Veronica Pratt, have found for the first time that other common stars in the galaxy also exhibit sympathetic stellar flares. They reported their findings recently in the Astrophysical Journal.
“This is the first time that an effect well-known on the Sun—sympathetic flaring—has been seen on other stars,” says David Martin, assistant professor of astronomy and physics, who collaborated on the research.
The research started as a class project trying to measure stellar flares to see if sympathetic flares existed outside of our solar system. To start, the team gathered data on more than 16,000 stars from the Transiting Exoplanet Survey Satellite, a NASA effort to look for planets that might support life across many distant galaxies.
A Novel Algorithm
Collaborators Jason Reeves, A23, AG25, and Andy Zhang, A26, developed an algorithm to detect sympathetic flares on those stars. Called TOFFEE (Threshold-Optimized Flare Finding and Energy Estimation), the algorithm parsed the data to figure out when flares were random, and when they were related to earlier flares.
“Flares begin very quickly, but they take a while for their brightness to fade,” says Pratt. “While one flare is fading, another one—related or not—can occur. That sequence makes it look like the star got brighter, a little dimmer, and then brighter again. That can be really hard for some traditional flare-detection algorithms to detect.”
The TOFFEE algorithm can distinguish between flares in close sequence, and judge—“with a lot of complicated mathematics,” says Pratt—if the second flare is a sympathetic one, related to the first. “It is analogous to someone yawning and then someone else in the room yawning right afterwards—was this a response mechanism or were they simply both tired?” says Martin.
It took “a lot of complicated code to be able to detect a second flare while the first one was still fading away,” says Pratt.
The team ran the algorithm on more than 200,000 flares on some 16,000 stars, and the results were striking. It turned out that the rate of sympathetic flares was between about 4% and 9%, similar to the 5% rate observed on our Sun. The sympathetic flares were also typically separated from the first flares by a half hour to one-and-a-half hours, similar to the timing seen on the Sun.
Most sympathetic flares occurred on stars called M dwarfs—“the smallest, coolest, and most common stars in the galaxy,” says Pratt. “It was an unexpected result, because M dwarfs, which made up the bulk of the sample, are so different from the Sun. They’re substantially smaller, half as warm, and substantially more active.”
The study results show that sympathetic flares occur on different types of stars at a roughly similar rate as they do to the Sun. “That implies to us that there’s a common mechanism across all stars that leads to sympathetic flaring,” says Pratt. “No one is sure exactly what that mechanism is yet, but it has to be something that is present on all kinds of stars, with different kinds of magnetic fields.”
