Fast Radio Bursts from Magnetars

September 01, 2020

The origin of mysterious fast radio bursts (FRBs) has been debated since their discovery in 2007. A theory developed at Columbia University and MPA suggested that FRBs are emitted by blast waves from flaring magnetars -- neutron stars with ultrastrong magnetic fields. On 28 April 2020, an FRB was detected from SGR 1935+2154, a known magnetar in our Galaxy. A new numerical experiment demonstrates how perturbations can grow in a magnetar and launch a magnetic explosion – and a burst such as the observed one.

Two FRBs (orange curve, observed by CHIME) detected during the X-ray burst (black curve, observed by INTEGRAL) from the galactic magnetar SGR 1935+2154 on 28 April 2020.

Fast radio burst (FRBs) last only a few milliseconds, but their extreme brightness makes them detectable from cosmological distances. Since 2016, after the first FRB localization, host galaxies have been identified for several FRB sources. Some sources were found to repeat, sometimes producing hundreds of bursts. One such repeater (FRB180916.J0158+65) demonstrates a 16-day period in its activity.

What could produce these bursts? The short duration of FRBs indicates a compact source, and neutron stars seemed suitable candidates; however, it proved challenging to identify a concrete emission mechanism. One hypothesis was that the bursts come from magnetars. About 30 such objects are known in our Galaxy, with ages of thousands of years. The hallmark of magnetars is their powerful X-ray flares. However, until 2020 none of them was seen to produce a radio burst.

The theory developed at Columbia University and MPA suggested that younger, hyper-active magnetars exist in distant galaxies. They flare frequently and launch relativistic blast waves into the persistent wind from the neutron star. These blast waves are capable of producing coherent radio emission by a maser-type instability. The instability develops at the shock front of the explosion and generates GHz radiation when the shock expands beyond 1 AU.

Numerical simulation of a magnetic explosion. Magnetic energy becomes concentrated in two shells ejected from the magnetar, launching two relativistic blast waves.

A direct test for the magnetar model is the detection of a radio burst from a known magnetar. This happened on 28 April 2020, when a mega-Jansky radio burst with a duration of a few milliseconds was detected from SGR 1935+2154, a magnetar in the Milky Way. Two radio telescopes, the Canadian CHIME and the STARE2 at Caltech, detected the burst. Data analysis revealed two FRBs, about 30 millisecond apart, which were both emitted during a 0.5-second X-ray burst. The two radio bursts nearly coincided with two narrow, bright spikes in the X-ray emission. 

This discovery has established the FRB-magnetar association, and also posed a new question. The FRBs from SGR 1935+2154 are intrinsically weaker than the previously detected cosmological FRBs. Astrophysicists previously thought that such low-energy events were incapable of launching a magnetic explosion. So, are the weak and strong FRBs produced by the same mechanism?

A precessing magnetar bursts in a direction that oscillates with time. Its observed FRB activity is modulated with the precession period.

An answer is suggested by a new numerical experiment performed in collaboration with Flatiron Institute in New York. The simulation followed the response of the neutron star magnetosphere to a low-energy perturbation generated by a quake of the magnetar. It shows that the perturbation propagates outward through the magnetosphere, grows in amplitude, and successfully launches a magnetic explosion at about hundred stellar radii. The result suggests that the same emission mechanism can work across a broad range of FRB luminosities, from the weak bursts of SGR 1935+2154 to the brightest cosmological FRBs.

Hyper-active magnetars – the putative cosmological FRB repeaters – are rather extreme objects. Their internal temperatures are about one billion degrees, and their cores are likely non-superfluid (in contrast to normal neutron stars). Their shapes are slightly non-spherical, because they are deformed by the ultrastrong magnetic fields. This leads to an interesting observational effect: the hyper-active magnetar can develop free precession with a period of weeks to months. It is consistent with the 16-day period observed in FRB180916.J0158+65.

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