Fast radio bursts were an enigma when they were first spotted. At first, each FRB followed the same pattern: a huge surge of energy in radio wavelengths that lasted less than a second—and then the burst was gone, never to repeat. We initially suspected FRBs might be hardware glitches in our detectors, but over time, the bursts’ recurrence convinced us that they were real.
Since then, we’ve identified sources of repeated bursts and associated the FRBs with a source that produces energy outside the radio range. This ultimately helped us point the finger at a single source: magnetars, or neutron stars that have extremely intense magnetic fields.
Now, reality has gone and thrown a monkey wrench in that nice and simple explanation. A new repeating source of FRBs has been identified, and it resides in a location where we wouldn’t expect to find any magnetars. This doesn’t mean that the source isn’t from a magnetar, but we have to resort to some unusual explanations for its formation.
Spinning neutrons
A magnetar is a form of neutron star, which is what’s left after the collapse of a star that is massive enough to generate a supernova but not massive enough to form a black hole. As that remnant is compressed into a soup of neutrons, the matter of a neutron star shrinks until it’s only about 20 kilometers across. That compact object inherits all the rotational energy of its parent star, causing it to spin at a rapid rate, often furthered by the addition of matter falling in from its environment.
In many cases, this rapid rotation results in pulsars, neutron stars that have sources of radiation that appear to blink rapidly in time with the star’s rotation. In some others, the neutron star ends up with an intense magnetic field, making it a magnetar. A magnetar’s intense magnetic field lines are whipped around by its rotation, often creating high-energy interactions with its environment.
But these high-energy phenomena don’t tend to last long, at least in astronomical terms. All of these energetic interactions with the environment cause the neutron star to shed energy, slowing its rotation and reducing the intensity of any light it produces. For example, magnetars are thought to typically have life spans on the order of only 10,000 years before fading into a quieter existence.
In addition, the supernova that form magnetars occur in relatively young stars, typically only a few million years old.
This combination—an early stellar death and a short magnetar life span—means we only expect to see magnetars in areas with an abundance of young stars. Older star populations should have seen magnetars form and fade out billions of years earlier.
Where was that from?
The new work, done by a large international team, involved following up on the discovery of another repeating FRB source, called FRB 20200120E. To identify where FRB 20200120E was located, the team turned to the resolving power of the European Very Long Baseline Interferometry Network, which can use as many as 22 telescopes scattered throughout the world. The team managed to get enough of those telescopes pointed at the repeating source to image five individual FRBs.
The way reconstructing data from these different telescopes work, a single burst will not get us a precise location. Instead, a range of possible locations can be identified. By combining the locations that are consistent with each of these bursts, the researchers were able to provide a likely location for the FRB source.
That source turned out to be a globular cluster of stars in the nearby galaxy M81. Based on the remaining uncertainty regarding the location of FRB 20200120E and the frequency of globular clusters within M81, the research team estimates that the chances of FRB 20200120E not being in this globular cluster is only about 1 in 10,000.
Searching that location did not reveal a persistent source of radio signals. No high-energy sources, based on searches using X-ray and gamma-ray telescopes, turned up either. So, there’s not an obvious high-energy object there.
What’s old is new again?
This location is odd. Globular clusters are most notable for consisting of populations of old stars. There are unlikely to have been any neutron-star-forming supernovae in them for billions of years. So that should probably rule the presence of a magnetar out, right?
Not entirely. A handful of mechanisms could produce a magnetar either without a supernova or long after one took place. These mechanisms mostly rely on a nearby companion star. If the companion is a normal star, it can feed matter into a white dwarf star until the dwarf collapses into a neutron star. Or various combinations of white dwarfs and neutron stars can merge, also producing a neutron star. Finally, we know that a normal companion can “spin up” a previously quiet neutron star by feeding it matter.
Any of these processes could potentially produce a magnetar within a population of old stars. Which process—if any of them—has actually taken place at FRB 20200120E may be difficult to sort out, given the apparent absence of any nonburst activity from the site.
In any case, the finding suggests that, should magnetars be the source of all FRBs, then we might expect to see them in a much broader range of environments than would have been predicted prior to this discovery. And we might not want to rule out consideration of nonmagnetar sources just yet.
Nature, 2022. DOI: 10.1038/s41586-021-04354-w (About DOIs).
