The exact origins of cosmic phenomena known as fast radio bursts (FRBs) are not fully understood, but scientists at the Massachusetts Institute of Technology (MIT) in the US have identified a fresh clue: at least one of these puzzling cosmic discharges got its start very close to the object that emitted it. This result, which is based on measurements of a fast radio burst called FRB 20221022A, puts to rest a long-standing debate about whether FRBs can escape their emitters’ immediate surroundings. The conclusion: they can.
“Competing theories argued that FRBs might instead be generated much farther away in shock waves that propagate far from the central emitting object,” explains astronomer Kenzie Nimmo of MIT’s Kavli Institute for Astrophysics and Space Research. “Our findings show that, at least for this FRB, the emission can escape the intense plasma near a compact object and still be detected on Earth.”
As their name implies, FRBs are brief, intense bursts of radio waves. The first was detected in 2007, and since then astronomers have spotted thousands of others, including some within our own galaxy. They are believed to originate from cataclysmic processes involving compact celestial objects such as neutron stars, and they typically last a few milliseconds. However, astronomers have recently found evidence for bursts a thousand times shorter, further complicating the question of where they come from.
Nimmo and colleagues say they have now conclusively demonstrated that FRB 20221022A, which was detected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in 2022, comes from a region only 10 000 km in size. This, they claim, means it must have originated in the highly magnetized region that surrounds a star: the magnetosphere.
“Fairly intuitive” concept
The researchers obtained their result by measuring the FRB’s scintillation, which Nimmo explains is conceptually similar to the twinkling of stars in the night sky. The reason stars twinkle is that because they are so far away, they appear to us as point sources. This means that their apparent brightness is more affected by the Earth’s atmosphere than is the case for planets and other objects that are closer to us and appear larger.
“We applied this same principle to FRBs using plasma in their host galaxy as the ‘scintillation screen’, analogous to Earth’s atmosphere,” Nimmo tells Physics World. “If the plasma causing the scintillation is close to the FRB source, we can use this to infer the apparent size of the FRB emission region.”
According to Nimmo, different models of FRB origins predict very different sizes for this region. “Emissions originating within the magnetized environments of compact objects (for example, magnetospheres) would produce a much smaller apparent size compared to emission generated in distant shocks propagating far from the central object,” she explains. “By constraining the emission region size through scintillation, we can determine which physical model is more likely to explain the observed FRB.”
Challenge to existing models
The idea for the new study, Nimmo says, stemmed from a conversation with another astronomer, Pawan Kumar of the University of Texas at Austin, early last year. “He shared a theoretical result showing how scintillation could be used a ‘probe’ to constrain the size of the FRB emission region, and, by extension, the FRB emission mechanism,” Nimmo says. “This sparked our interest and we began exploring the FRBs discovered by CHIME to search for observational evidence for this phenomenon.”
The researchers say that their study, which is detailed in Nature, shows that at least some FRBs originate from magnetospheric processes near compact objects such as neutron stars. This finding is a challenge for models of conditions in these extreme environments, they say, because if FRB signals can escape the dense plasma expected to exist near such objects, the plasma may be less opaque than previously assumed. Alternatively, unknown factors may be influencing FRB propagation through these regions.
A diagnostic tool
One advantage of studying FRB 20221022A is that it is relatively conventional in terms of its brightness and the duration of its signal (around 2 milliseconds). It does have one special property, however, as discovered by Nimmo’s colleagues at McGill University in Canada: its light is highly polarized. What is more, the pattern of its polarization implies that its emitter must be rotating in a way that is reminiscent of pulsars, which are highly magnetized, rotating neutron stars. This result is reported in a separate paper in Nature.
In Nimmo’s view, the MIT team’s study of this (mostly) conventional FRB establishes scintillation as a “powerful diagnostic tool” for probing FRB emission mechanisms. “By applying this method to a larger sample of FRBs, which we now plan to investigate, future studies could refine our understanding of their underlying physical processes and the diverse environments they occupy.”
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