During a three-week test run, the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a revolutionary radio telescope with no moving parts, has picked up 13 unexplained radio signals coming from beyond the Milky Way. These signals, known as fast radio bursts (FRBs) due to how quickly they come and go, are part of one of the strangest mysteries of modern astronomy, but the new detections could help unlock their source, thanks to a rare one that seems to be repeating.
Located in British Columbia, CHIME is a completely new type of radio telescope. Rather than physically moving dishes to point at different targets in the sky, CHIME has four metal troughs, each 100 m (330 ft) long, that can focus its attention digitally.
"The telescope has no moving parts," says Kiyoshi Masui, a member of the collaboration behind the new studies. "Instead it uses digital signal processing to 'point' the telescope and reconstruct where the radio waves are coming from. This is done using clever algorithms and a couple of giant computer clusters that sit beside the telescope and crunch away at the data in real time."
Fast Radio Bursts
One of the main uses planned for CHIME is to investigate FRBs, and the telescope proved its worth by detecting 13 of them in just three weeks in mid-2018. Even more impressive is the fact that this was during a pre-commissioning phase, meaning CHIME was running at just a fraction of its eventual capacity.
The first FRB was discovered in 2007 as an anomaly in archival data, but the phenomenon wasn't detected live until 2015. Since then the total has climbed to about 60 events, with each one bringing new clues as to their origin.
A burst known as FRB121102 is by far the most bizarre. Instead of being a one-hit wonder like all the others, this FRB has been seen to pulse over 100 times since its discovery in 2012, sometimes laying dormant for months before flaring back up and releasing dozens of bursts within the space of a few hours. Until now FRB121102 was the only repeater, but one of the 13 new FRBs discovered by CHIME seems to be doing the same thing, which could be key to unlocking the mystery.
"Until now, there was only one known repeating FRB," says Ingrid Stairs, a member of the CHIME team. "Knowing that there is another suggests that there could be more out there. And with more repeaters available for study, we may be able to understand these cosmic puzzles a bit better — where they're from, what causes them, and why."
But the repeater wasn't the only breakthrough. Most other FRBs are usually detected at frequencies of about 1,400 MHz, but CHIME's range is between 400 and 800 MHz. That makes these new events the lowest-frequency FRBs found so far, and the team says that finding bright signals towards the bottom of that range suggests that future detections could be found even lower than 400 MHz.
Because of its lower frequency operations, CHIME can also make more precise measurements of the scattering of the radio waves. The team observed a large amount of scattering, which suggests that whatever astrophysical objects are emitting the signals, they're probably close to a lot of interference of some sort.
"That could mean in some sort of dense clump like a supernova remnant," says Cherry Ng, a member of the CHIME team. "Or near the central black hole in a galaxy. But it has to be in some special place to give us all the scattering that we see."
Interestingly, an unrelated study also published today backs up these observations.
Milky Way magnetar
One of the most commonly-suggested culprits behind FRBs is a magnetar, a type of neutron star with an extremely strong magnetic field. As these objects spin, they give off pulses of signals.
To investigate, researchers from Caltech and JPL examined a magnetar called PSR J1745-2900, located in the galactic center of the Milky Way. And if you know your galactic geometry, that's a turbulent neighborhood – this magnetar sits just 0.3 light-years from the supermassive black hole Sagittarius A*.
Sure enough, the team saw that the signals given off by the magnetar looked a lot like those of FRBs.
"Our observations show that a radio magnetar can emit pulses with many of the same characteristics as those seen in some FRBs," says Aaron Pearlman, an author of the study. "Other astronomers have also proposed that magnetars near black holes could be behind FRBs, but more research is needed to confirm these suspicions."
With each new detection, the mystery of FRBs may get a little closer to being solved.
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