As mysterious as fast radio bursts (FRBs) are, they’re now so common that they’re at risk of becoming mundane. But a newly discovered signal deepens the mystery with a few oddities – it hails from an unexpected region of space, and its pulses are about a million times shorter than most, which could indicate many others like it are going undetected.
Fast radio bursts are very true to their name – they’re energetic bursts of radio signals from deep space that last just milliseconds. Thousands of FRBs have been detected since they were first identified in 2007, with some being one-time events and others repeating either randomly or in a predictable rhythm. While their origins are still unclear, each new detection adds more clues – and this latest find brings a lot to the table.
In January 2020, a repeating signal was detected from the constellation of Ursa Major. For the new study, astronomers investigated its source using 12 parabolic antennas from the EVN observation network. They were able to track the FRB to the edges of the spiral galaxy Messier 81, located about 12 million light-years from Earth. That might sound like a long way off, but it’s a cosmic stone’s throw away compared to the many millions or billions of light-years that most FRBs travel to reach us.
Within that galaxy, the signal was coming from a globular cluster, a dense group of ancient stars – and that’s surprising, because most FRBs have been found in areas where the stars are a lot younger. The lead suspect for what’s behind FRBs is a type of star known as a magnetar, a small, dense, highly magnetized core left over after a massive star explodes as a supernova. But these magnetars should be very rare in globular clusters.
"Strange things happen over the course of a globular cluster's several billion years of existence,” said Franz Kirsten, co-lead author of the study. “We suspect that we are looking at a star with an unusual history.”
This wouldn’t be a run-of-the-mill magnetar – the team hypothesizes that the object in question was once a white dwarf, in a binary system. As it orbited its partner closely, it began to slurp material off the other star, until it gained too much mass and collapsed into a magnetar. Although this scenario would be rare, the team says it’s the easiest way to produce fast radio bursts in a globular cluster. Intriguingly, this could be the first evidence of a magnetar born from a white dwarf, something that has only been theoretically described so far.
On closer inspection, the team found other oddities to the signals. While most FRB chirps last on the scale of milliseconds, some of these only lasted a few dozen nanoseconds, which are a million times shorter. That suggests that the object behind them is absolutely tiny – perhaps only a few dozen meters wide, as compared to the usual 10 km (6 miles) or so.
The researchers suggest that this could indicate there’s a whole other category of ultra-fast FRBs out there, which current instruments aren’t listening out for.
The research was published in two papers – one focusing on the source’s location in a globular cluster was published in the journal Nature, while another discussing the ultrafast pulses appeared in Nature Astronomy.
Source: Max Planck Institute