Very-high-frequency oscillations in the main peak of a magnetar giant flare.

Magnetars are strongly magnetized, isolated neutron stars 1-3 with magnetic fields up to around 10 15  gauss, luminosities of approximately 10 31 -10 36  ergs per second and rotation periods of about 0.3-12.0 s. Very energetic giant flares from galactic magnetars (peak luminosities of 10 44 -10 47  ergs per second, lasting approximately 0.1 s) have been detected in hard X-rays and soft γ-rays 4 , and only one has been detected from outside our galaxy 5 . During such giant flares, quasi-periodic oscillations (QPOs) with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been observed 6-9 , but their statistical significance has been questioned 10 . High-frequency QPOs have been seen only during the tail phase of the flare 9 . Here we report the observation of two broad QPOs at approximately 2,132 hertz and 4,250 hertz in the main peak of a giant γ-ray flare 11 in the direction of the NGC 253 galaxy 12-17 , disappearing after 3.5 milliseconds. The flare was detected on 15 April 2020 by the Atmosphere-Space Interactions Monitor instrument 18,19 aboard the International Space Station, which was the only instrument that recorded the main burst phase (0.8-3.2 milliseconds) in the full energy range (50 × 10 3 to 40 × 10 6  electronvolts) without suffering from saturation effects such as deadtime and pile-up. Along with sudden spectral variations, these extremely high-frequency oscillations in the burst peak are a crucial component that will aid our understanding of magnetar giant flares.