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For more than a decade, scientists have been working to unravel the mysteries of so-called “fast radio bursts” (FRBs), which are sudden and unexplained radio pulses that are mostly observed in distant galaxies.
These bizarre bursts, which last for mere milliseconds and sometimes repeat in odd patterns, have inspired explanations ranging from pyrotechnic interactions between exotic stars to signs of extraterrestrial intelligence.
Now, astronomers have managed to probe a repeating burst at the shortest timescales ever, meaning that a team studied its signature at tiny periods of just 3 to 4 microseconds within those millisecond pulses. This unprecedented analysis revealed a newly observed “microstructure,” or a pattern of variable brightness, revealing that the new technique “can unveil clues into [FRB] emission physics,” according to a study published on Monday in Nature Astronomy.
A team led by Kenzie Nimmo, a PhD student at the Anton Pannekoek Institute for Astronomy at the University of Amsterdam, obtained this “high-time-resolution” data from the European Very Long Baseline Interferometry Network, a vast network of radio telescopes that spans four continents. Their target was FRB 180916, a curious repeating burst that operates on a cycle of 16 days; it actively bursts for four days and goes quiet for 12 days.
“The microstructure that we refer to in the [study’s] title is that we see the brightness of the burst itself vary on microsecond timescales,” said Nimmo in an email. She also noted that the polarized properties of the burst also fluctuate at the microsecond level.
The results are interesting, Nimmo added, because these “short timescale brightness variations strongly constrain the size of the FRB emission region, which dictates what models can work for producing FRBs.”
In other words, studying an FRB at very short timescales provides a means of zooming into the physical space around the unknown source of these radio pulses. The microsecond resolution achieved by Nimmo and her colleagues enabled them to determine that the size of the emission region, meaning the area that is creating these pulses, is about a kilometer (0.62 miles) in scale.
What makes this finding so impressive is that the source of the burst is located some 457 million light years away from Earth. While that actually makes FRB 180916 relatively close compared to other FRBs, it is still mind-boggling Nimmo’s team has captured kilometer-scale details from such an enormous intergalactic distance.
Scientists have previously captured details of FRBs at timescales of about 20 to 30 microseconds, which makes the new study about 10 times more precise than anything before it. At this resolution, the team could make out interesting details about the “polarization position angle” (PPA), which is the angle at which the burst’s polarized light oscillates. This property is important for figuring out details about the FRB source’s spin, and how close the radio emission is to its source, which can in turn shed light on its possible identity.
“We noticed that on very short timescales (less than 100 microseconds or so), we see small variations in the PPA,” Nimmo said. “This could mean that we are starting to resolve the rotation of whatever object is producing FRBs.”
Based on their results, the researchers think that neutron stars, a type of extremely dense dead star, are “the most compelling progenitor model for FRBs,” Nimmo added. These roiling balls of compact matter contain more mass than the Sun, yet they are only about 12 miles in diameter.
As a consequence, neutron stars are extremely volatile and could be capable of the type of extreme radio outbursts seen in FRBs. Indeed, a weak radio burst within our own galaxy, the Milky Way, was traced back to a special type of highly magnetized neutron star located about 30,000 light years from Earth.
Some models of FRBs suggest that their radio pulses originate close to the star, within its magnetosphere, while others suggest that the emission is the result of a relativistic shock that occurs further away from the source, Nimmo said. This new high-time-resolution study favors the former scenario, in which the emission emerges close to the neutron star.
The repeating periodicity of FRB 180916, meanwhile, hints that it may originate in a binary system that contains a precessing (wobbling) neutron star and a massive star that share an orbital period of 16 days. When these objects are closest to each other during this orbit, interactions between them could amplify the bursts, causing the four-day period of activity that we see on Earth.
Ultimately, Nimmo and her colleagues hope to study these mysterious bursts at even shorter timescales, though she notes that pushing these temporal limits will involve a host of observational and data-volume challenges. That said, the payoff could be huge: it’s possible that this line of research could reveal that one-off FRBs might actually be repeaters, assuming scientists can look at them closely enough, in addition to a range of other possible breakthroughs.
“In our study we measure a range of timescales from microseconds to milliseconds and suggest that this might be characteristic of repeating FRBs, so searching for this range of timescales in the future could be a way to identify a repeating FRB (from a non-repeating FRB),” Nimmo said.
“This is very important as it is still debated whether repeating and non-repeating FRBs are the same thing, or if they come from different progenitors/emission physics,” she added. “Having identifiers to distinguish between them, as opposed to simply observing multiple bursts, is invaluable both for our understanding of them, but also to help future observations be more lucrative by studying sources that ‘look’ like repeaters.”