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.
(© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)