To catch an FRB
And hunting down Milky Way FRBs is exactly what researchers attempted with the Five Hundred Meter Aperture Spherical Telescope (FAST) in China back in April.
The international team of astronomers used the new, massive, $180-million instrument to keep an eye on the magnetar SGR 1935+2154 in hopes of seeing it produce an FRB. FAST is capable of studying small areas of the night sky and can detect even very faint radio signals. This is what made the FAST team a clear favorite to be first to detect an FRB in the Milky Way. And throughout April, they tuned into the magnetar as other telescopes observed it repeatedly ejecting X-rays bursts.
Then, on April 27, NASA’s orbiting Neil Gehrels Swift Observatory started to detect gamma-rays coming from the magnetar. Astronomers’ models had suggested that magnetars emit such signals just before an FRB. And sure enough, just a day later, a half-pipe shaped radio observatory in Canada called the Canadian Hydrogen Intensity Mapping Experiment caught a glancing blow from an FRB.
They weren’t the only ones to see it, either.
Christopher Bochenek is a graduate student at Caltech. For his observation, he had used cake pans and some steel pipe to construct a radio observatory called STARE2 across three sites in Utah and California. Bochenek was hoping to catch an FRB from inside our Milky Way, but he says his group figured they had about a 1-in-10 chance of actually pulling it off.
After CHIME’s FRB detection, Bochenek checked STARE2 to see if he’d beaten the odds.
“When I looked at the data the first time, I froze and was basically paralyzed with excitement,” Bochenek said. “It took me a few minutes to collect myself.”
It was the brightest radio signal ever detected from within our galaxy. For comparison, the previous record holder for brightest burst of radio waves in our galaxy was a pulsar in the Crab Nebula. This FRB, however, was thousands of times brighter than the Crab's radio burst, his team reported in Nature.
“This burst was so bright that, in theory, if you had a recording of the raw data from your cell phone’s 4G LTE receiver, which does detect radio waves, and if you knew what you were looking for, you might have found this signal that came about halfway across the galaxy in your cell phone data,” Bochenek said.
Yet his telescope was the only one on Earth to catch the FRB head on. Despite its observations of the magnetar, FAST didn’t detect the actual FRB.
But that raises another intriguing question: Why didn’t the Chinese FAST telescope see this FRB when it’s capable of witnessing signals eight times fainter than either of the other observatories? Researchers think the answer could be that their telescopic gaze was too narrow, so FAST simply missed it.
Whatever the reason, astronomers think there’s something to be learned about FRBs from the fact that FAST was pointed directly at the magnetar yet saw nothing, even as it erupted with X-rays and gamma rays. What exactly is that lesson? Astronomers will have to keep a close eye on the sky to find out.