magnetar

Repeated bursts of radio waves from nearby stars suggest that such signals are common

By Jonathan O’Callaghan

Artist’s impression of the SGR 1935 + 2154 magnetar

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Mysterious Fast Radio Bursts (FRBs), short and powerful explosions of radio waves in space, may not be quite as rare and unusual as we thought. Astronomers have discovered that a star in our galaxy that produces FRBs can also produce weaker but more frequent bursts.

“What we are showing here is that FRBs can lose much more brightness than we thought,” says Franz Kirsten from Chalmers University of Technology in Gothenburg, Sweden. “We conclude that FRBs are probably much more common than we think.”

Earlier this month, researchers said they had found an FRB source in our galaxy for the first time. The conclusion was based on observations made in April by the CHIME telescope in Canada and the STARE2 radio receivers in California and Utah. Astronomers suspected that the FRB source was a magnetar – a neutron star with a strong magnetic field – that was about 30,000 light years from Earth.

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Interestingly, a few days after these observations, another team used the FAST telescope in Guizhou, China to capture an impulse from the magnetar called SGR 1935 + 2154, which was a million times weaker than the original FRB.

Now Kirsten and his colleagues say they discovered two more weak outbreaks of SGR 1935 + 2154, each lasting just a millisecond and 1.4 seconds apart. The weaker outbreaks were detected by the Westerbork Synthesis Radio Telescope in the Netherlands in May. Both of the May outbreaks were about 10,000 times weaker than the original April FRB.

“The exciting thing is that these bursts bridge the gap between single pulses, which are quite weak, and FRB-like bursts,” says Chris Bochenek of the California Institute of Technology, who led part of the first discovery of the FRB source in April. “We [now] I know there are processes throughout this spectrum of energy. “

An FRB is loosely defined as any radio burst bright enough to be seen from another galaxy. However, it is possible that these weaker bursts – only visible to astronomers because their source is relatively close to Earth – could be generated by the same mechanism as FRBs. They can even be FRBs, albeit weaker ones.

The mechanism of FRB production is currently unknown. “There are many theories,” says Daniele Michilli, a member of the CHIME team based at McGill University in Montreal, Canada. “In the first class, the emission comes from the star’s magnetosphere. In a second class of theories there is like a ball of fire that ignites the emission of a plasma cloud further away from the star. “

With more observations of magnetars like SGR 1935 + 2154, it may be possible to see more evidence of different areas of burst activity. Factors like the age of the star could play a role, say Kirsten and colleagues, with younger magnetars producing brighter FRBs. Other cosmic objects such as binary stars could produce FRBs in addition to magnetars.

“This paper raises new questions,” says Bochenek. “Is that like an FRB emission? Where do you do [emission] stop and start the other? And what is an FRB? “

Journal reference: Nature Astronomy, DOI: 10.1038 / s41550-020-01246-3

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