© ASA, ESA and the Hubble Heritage Team (STXCI/AURA)
Often originating billions of light-years away, the extremely bright, extremely brief bursts of radio waves known as fast radio bursts, or FRBs, have defied explanation since they were first spotted in 2007. Based on observations to date, scientists surmised that the bursts are powered by young, short-lived cosmic objects called magnetars.
But a fast radio burst discovered last year has now been traced to a globular cluster about 11.7 million light-years away, near the neighbouring spiral galaxy M81, according to a paper describing the discovery posted on the scientific preprint server arXiv. Finding this burst among a cluster of ageing stars is kind of like finding a smartphone embedded in Stonehenge — the observation doesn't make sense.
"This is definitely not a place fast radio bursts are expected to live," Bryan Gaensler, an astronomer at the University of Toronto and a co-author of the new paper, posted on Twitter. "Just what is going on?"
Scientists are struggling to explain the cosmic anachronism. They're also moving toward the conclusion that maybe, as with many other celestial phenomena, there are multiple ways to cook up a fast radio burst.
"FRBs might be — might be — just this generic phenomenon associated with a whole range of possible sources," says Cornell University astronomer Shami Chatterjee, who studies the bursts but is not part of the discovery team.
"What is happening here?"
Scientists first spotted the burst, dubbed FRB 20200120E, in January 2020 using the Canada Hydrogen Intensity Mapping Experiment (CHIME) telescope, which has proven to be a relentless FRB-finding machine. When CHIME came online in 2017, scientists knew of fewer than 30 fast radio bursts; now the telescope has boosted that total to well over a thousand.
Like at least two dozen known bursts, FRB 20200120E is a repeater — a space engine that produces multiple detectable blasts of radio waves, rather than exploding once and vanishing. Its bursts are not as bright as those coming from billions of light-years away, in the distant cosmos, but over the last year, they've allowed scientists to identify the FRB's location in the sky.
From there, the team could attempt to identify a source. Measurements of the bursts suggested that FRB 20200120E was quite nearby, so astronomers knew they were hunting for something local, perhaps even within the Milky Way's gassy, sparsely populated halo. Scientists then used a network of radio telescopes known as the European Very Long Baseline Interferometry Network to pinpoint the burst's precise location.
"We conclusively prove that FRB 20200120E is associated with a globular cluster in the M81 galactic system, thereby confirming that it is 40 times closer than any other known extragalactic FRB," the authors write in the new paper.
"The interpretation of that is where things get very, very interesting," Chatterjee says. "It is very hard to fit into existing models."
Globular clusters are some of the most ancient objects in the observable universe. They're billions of years old, at least as old as the galaxies they orbit, and perhaps much older. Until now, scientists strongly suspected that fast radio bursts were produced by some of the youngest compact objects yet observed — magnetars, or extremely magnetic, flaring stellar corpses produced when young, massive stars explode and die. Once formed, the ultramagnetic stellar corpse lingers for tens of thousands of years before its magnetic field decays, leaving a more ordinary neutron star.
But as far as astronomers know, these sparkling, densely packed globular clusters don't contain the kinds of tempestuous stars that collapse into magnetars.
"This type of star formation is happening all around the universe, even in our own galaxy in many places, but not in globular clusters," says Northwestern University's Claire Ye, who studies globular clusters. "It's like, wow, what is happening here?"
Extremely magnetic, ultra-dense stars
It's taken nearly 15 years to begin untangling the mystery of fast radio bursts. Initial hypotheses included evaporating black holes, flaring dead stars, colliding dense objects, and yes, even alien technologies (spoiler: it's not aliens). Further clues, from nano-scale structures within the radio bursts to their millisecond duration and intensity, suggested they must be produced by extremely dense, compact objects.
So, scientists turned to objects such as black holes and neutron stars, which are left over when massive stars blow themselves to bits in supernovae. Later, observations suggested that some bursts are born in regions with extreme magnetic fields, further suggesting these mysterious signals could come from magnetars.
Then, last year, a magnetar within the Milky Way produced a radio burst resembling an FRB. The blast was a bit wimpier than the extremely powerful bursts coming from a half a universe away, but scientists were convinced they were on the right track.
"The paradigm that FRBs come from magnetars has taken on quite a life since we saw the FRB-like burst from the galactic magnetar," says Brian Metzger of Columbia University and the Flatiron Institute. "You had a situation where both the theorists and the observers were pretty happy with magnetars."
"Finding this burst among a cluster of ageing stars is kind of like finding a smartphone embedded in Stonehenge — the observation doesn't make sense. "
But that didn't last long. With the discovery of FRB 20200120E, astronomers now need to figure out how magnetars might arise and survive in globular clusters, or they need to figure out how a population of extremely old, quiet stars can generate such powerful blasts. Neither is an easy problem to solve.
Plausible explanations
While astronomers don't think globular clusters contain magnetars, other types of stellar corpses should be plentiful. White dwarfs, which are formed when sun-like stars balloon into red giants and die, and neutron stars, formed by larger supernovae, can be created early in the lives of these ancient clusters.
Perhaps magnetars can arise when two neutron stars collide and merge, when two white dwarfs collide and merge, or when a white dwarf with an orbiting companion star steals so much mass that it collapses into a newborn neutron star. So far, however, no one has seen a magnetar formed in these ways.
Northwestern University's Ye thinks we need to look at other ways to possibly form magnetars in these clusters, and to explore how other stars could power fast radio bursts. As well, she says, it's crucial to gather more information about this particular cluster to see what else could be creating the epic blasts.
"Globular clusters are different," she says. "Some are denser, some are less dense, and in different clusters you will see different outcomes."
Metzger also notes that it should be possible to generate something that looks like a fast radio burst in the absence of magnetars. Two neutron stars whirling around one another could generate outbursts that resemble fast radio bursts, as could turbulent discs of material swirling around black holes that occasionally produce jets and flares. "I'm a little more inclined to think there's something other than magnetars going on here," he says.
Chatterjee agrees, adding that "maybe some fraction of FRBs are not related to magnetars, but are instead related to some sort of black hole jet phenomenon."
Perhaps fast radio bursts are formed through multiple pathways — kind of like gamma-ray bursts, which confounded astronomers for decades after initially being discovered by a military satellite in the 1960s. Now, we know that both powerful supernovae and colliding neutron stars can produce these immensely energetic flashes of gamma rays.
"Nature found two ways to do that," Metzger says. "I think we may be seeing something similar with FRBs."
R.C.