Mysterious space signal could be neutron star near black hole

Dutch-led team suggests a new explanation to account for a transient astronomical phenomenon. Andrew Masterson reports.

The Arecibo Observatory in Puerto Rico, with which fast radio bursts from the same source have been detected 16 times.
The Arecibo Observatory in Puerto Rico, with which fast radio bursts from the same source have been detected 16 times.
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The source of a mysterious repeating fast radio burst emanating from a dwarf galaxy three billion light years from Earth could be a neutron star positioned close to a supermassive black hole, according to research published in the journal Nature.

Fast radio bursts (FRBs) are radio pulses that last mere microseconds and come from unknown sources in deep space. Their narrow structure and brief duration makes them extremely difficult to detect. The first to be recorded was in 2007.

The subject of the latest paper, FRB 121102, was first recorded in 2012 by astronomers at the Arecibo Observatory in Puerto Rico. Since then it has been detected another 15 times, and is the only confirmed repeating FRB.

The fact that FRB 121102 does repeat has already provided strong evidence that fast radio bursts are not produced by some sort of cataclysm, thus excluding early theories that they may the result of black hole or neutron star collisions.

The search for explanation continues, however, with some theories perhaps less likely than others. In 2017, for example, two Harvard astrophysicists wrote a paper that suggested the measurements of FRBs were consistent with “beams used for powering large light sails” by extragalactic civilisations.

In the latest research, a team led by Jason Hessels of the Netherlands Institute for Radio Astronomy produce a less sensational although still extraordinary explanation.

Analysing the 16 FRB 121102 signals recorded to date – all from the Arecibo Observatory – Hessels and his colleagues note that the millisecond radio flashes have been localised to a star-forming region of a dwarf galaxy and that they emanate from close to another phenomenon, described as “a compact, persistent radio source”.

The researchers note: “The origin of the bursts, the nature of the persistent source and the properties of the local environment are still unclear.”

There are some clues in the data, however, that narrow down the field.

The extremely variable rotation of the polarisation plane of the radio waves coming from the source of FRB 121102, the scientists report, together with their short duration and tendency to repeat on a frequency of about seven months, strongly suggest that whatever is producing the bursts exists in a locality marked by both extremely high temperatures and very strong magnetic fields.

These factors, they conclude, “suggest a neutron star origin”.

The variations in the polarisation plane – known as the Faraday rotation measure – are uncommon. Hessels and his colleagues note that so far they have only ever been recorded in in the vicinities of massive black holes with masses greater than 10,000 times that of the sun.

The persistent radio source coming from somewhere nearby the FRB, they add, is consistent with that “of a low-luminosity, accreting massive black hole”.

The most likely explanation for FRB 121102 is that its origin is a neutron star positioned close to a whopping black hole.

The data, however, is not sufficient to exclude other possibilities. The researchers note that it could be explained by a couple of other scenarios, “such as a highly magnetised wind nebula or supernova remnant surrounding a young neutron star”.

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Andrew Masterson is news editor of Cosmos.
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