Abstract:
Magnetars are strongly magnetized, isolated neutron stars1–3
with magnetic felds
up to around 1015 gauss, luminosities of approximately 1031–1036 ergs per second
and rotation periods of about 0.3–12.0 s. Very energetic giant fares from galactic
magnetars (peak luminosities of 1044–1047 ergs per second, lasting approximately 0.1 s)
have been detected in hard X-rays and soft γ-rays4
, and only one has been detected
from outside our galaxy5
. During such giant fares, quasi-periodic oscillations (QPOs)
with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been
observed6–9
, but their statistical signifcance has been questioned10. High-frequency
QPOs have been seen only during the tail phase of the fare9
. 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 fare11 in the direction of the NGC 253 galaxy12–17,
disappearing after 3.5 milliseconds. The fare was detected on 15 April 2020 by the
Atmosphere–Space Interactions Monitor instrument18,19 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 × 103
to 40 × 106
electronvolts)
without sufering from saturation efects 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 fares
