Mystery Repeating Radio Signal Traced to Strange V-Shaped Star-Forming Region of Massive Spiral Galaxy

A set of repeating radio signals coming from deep space have been traced back to a huge spiral galaxy around half a billion light years away. These fast radio bursts, or FRBs were also found to be coming from an "odd" V-shaped star-forming region of the galaxy—but what is producing them is unknown.

FRBs are powerful radio signals that last for just a few milliseconds. Because they are so short-lived, they are often only detected in astronomical data after the signal has passed Earth, so tracing them back to their original source is very difficult. Over 100 FRBs have been discovered over recent years and most of these have been one-off events, potentially suggesting they were caused by some cataclysmic event such as the collapse of a black hole or colliding neutron stars.

However, the discovery of repeating FRBs meant this explanation could not account for all of these radio signals.

The first repeating FRB, known as FRB 121102, was discovered in 2017 and was eventually traced back to a dwarf galaxy 3 billion light-years away. Then last year another repeating FRB—FRB 180916—was also discovered.

In a study published in Nature, an international team of scientists has now traced FRB 180916 back to its source galaxy, finding it is very different from the one where FRB 121102 originated. Had the two galaxies had similar properties, it may have helped narrow down what could be producing the radio bursts.

Study author Jason Hessels, from the Netherlands Institute for Radio Astronomy, told Newsweek FRB 180916 comes from a large spiral galaxy similar to the Milky Way. At half a billion light years away, it is "is by far the closest well-localised FRB to date—about six times closer compared to FRB 121102." It has about 100 times more stars than the galaxy FRB 121102 came from. "Whereas FRB 121102 had an associated persistent radio source—which has been hypothesized to represent a surrounding nebula or nearby black hole—we see no such source near FRB 180916," he added.

Instead, FRB 180916 appears to be coming from an outer spiral arm and is "associated with a strangely 'v'-shaped star-forming region," Hessels said. "We know that this is a star-forming region because we measured ionized gas at that location—and the source of ionization (energy) is consistent with coming from young stars. "The 'v'-shape of this region is quite odd ... one speculation is that it could be a disrupted satellite dwarf galaxy of the spiral galaxy. If so, that could be a way to make the situation more similar to what we saw with FRB 121102, but currently that's a very shaky hypothesis."

What could be producing these radio bursts is a mystery. "I'm really scratching my head," he said. " For FRB 121102, a young magnetar—a super-magnetized neutron star—appeared like a very plausible model. For FRB 180916 a somewhat older magnetar could still fit the data, but I'm ... wondering if maybe a different source of energy e.g. the interaction between two nearby objects, like the jet of a black hole impinging on the strong magnetic field of a neutron star, couldn't be the answer.

"That scenario has also been hypothesized for FRB 121102 but was previously considered less plausible by many."

He added that the radio source of FRB 121102 was thought to have been an extremely energetic nebula surrounding it. "The lack of persistent radio source with FRB 180916 is quite puzzling ... Maybe FRB 180916 had such a nebula (and radio source) but it has faded away because the source is much older."

Hessels said the team is hoping to use the Hubble Space Telescope to look at the position of FRB 180916 more closely. "That's actually one of the most exciting things about this source: since it's so nearby we can dig deep with our best optical, X-ray and gamma-ray telescopes and try to distinguish between competing models by detecting emission at other wavelengths," Hessels said.

FRB galaxy
Image showing the position of the repeating FRB coming from a relatively nearby spiral galaxy. The small arrow shows how the source appears to be on the arm of the galaxy. Shriharsh Tendulkar / Gemini Observatory