Photographing a lightning bolt at the exact moment it strikes can be considered a stroke of luck. Capturing the beginning of an explosion on the surface of a star presents a similar difficulty for astronomers, as these events are both brief and unpredictable. The scientific community, however, constantly strives to capture these fleeting moments, in order to better understand the mechanisms of the most energetic cosmic phenomena.
In November 2024, a space instrument, the SVOM/GRM, achieved an exceptional feat by successfully detecting a superflare, a gigantic eruption, on the star HD 22468 at the very moment of its trigger. This observation is a first for an RS CVn-type star, thus providing a direct view of these explosions of rare violence.
Artist's illustration of an eruptive solar storm.
Credit: NASA's Goddard Space Flight Center/Genna Duberstein A superflare corresponds to a sudden release of energy on the surface of a star, with an intensity much greater than the flares of our Sun. In the space of a few minutes, it can emit the equivalent of several months of the energy normally emitted by the star. These extreme events temporarily make the star shine with increased intensity, particularly in high-energy wavelengths like X-rays.
RS CVn-type stars are generally binary systems where two stars orbit very close to each other. This proximity generates particularly strong magnetic interactions, making these stars remarkably active. Their corona, an extremely hot region, accumulates magnetic tensions that are released violently during superflares.
Analysis of the collected data showed temperatures reaching up to 180 million degrees Fahrenheit (100 million degrees Celsius). The peak of hard X-ray emission came first, followed by a more prolonged emission in soft X-rays and visible light. This chronology of events helps reconstruct the way energy dissipates during these explosions.
These precise measurements help improve computer models describing the behavior of stars. A better understanding of superflares also helps assess their potential impact on surrounding exoplanets, particularly regarding their habitability.