Magnetar's Immense Explosion Reveals High Frequency Oscillations For First Time

Artist's impression of a gamma ray burst caused by an exploding magnetar - a sort of neutron star with an extraordinarily powerful magnetic field. Image Credit ESO.

In slightly more than a tenth of a second the magnetar explosion named GRB 200415A released more energy than the Sun does in 100,000 years. Within this outburst astronomers observed exceptionally short oscillations in brightness, the first time scientists have witnessed these fast changes within a neutron star eruption. The results come not from a telescope designed for the purpose, but one intended to study Earth's atmosphere from above.

Magnetars are a small subpopulation of neutron stars, of which only about 30 are known. They combine the density of other neutron stars (the densest objects in the universe outside black holes) with magnetic fields of at least a hundred billion Teslas, around 10 billion times the magnets in the Large Hadron Collider. They are known to experience eruptions lasting a few tenths of a second, but these have usually saturated observing equipment, preventing detailed study.

GRB 200415A was seen in April 2020 and announced based on observations with the Fermi Gamma-ray Space telescope. Extra information has been added from an unexpected source, the Atmosphere Space Interactions Monitor (ASIM) on the International Space Station, and published in Nature. The observations have been analyzed in detail for what they can tell us about matter in its most extreme state.

Rather than looking directly at space objects, the ASIM studies the Earth's atmosphere, looking for the "transient luminous events" that occur above thunderstorms, such as “elves”, “sprites” and “blue jets”. However, its field of view is large enough that it caught sight of the magnetar explosion out of the corner of its eye and provided more useful data than the purpose-built equipment.

Events like these are very difficult to observe in detail. "The difficulty lies in the brevity of the signal, whose amplitude rapidly decays and becomes embedded in background noise. And, as it is correlated noise, it is difficult to distinguish its signal," said Professor Victor Reglero of the University of Valencia in a statement. Most magnetar explosions have led to the observing equipment saturating, leaving us blind to short-term variations in brightness.

When all other instruments saturated, the ASIM did not, helped perhaps by GRB2001415 being the most distant magnetar eruption detected originating in a star-formation region of the galaxy NGC 253, in the Sculptor group. ASIM data reveals two quasi-periodic oscillations (QPOs) of 2,132 and 4,250 Hertz respectively.

"Seen in perspective, it has been as if the magnetar wanted to indicate its existence to us from its cosmic solitude, singing in the kHz with the force of a Pavarotti of a billion Suns," Reglero said. 

The paper attributes the QPOs to instabilities in the magnetosphere close to the surface of the star or a starquake in the magnetar's kilometer-deep crust. One of these triggered Alfvén waves in the magnetosphere, which bounced back and forwards between magnetic field lines, dissipating so much energy it saturated other instruments 13 million light-years away.

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