Rapid radio bursts were a mystery when they were first noticed. In the beginning, each FRB followed the same pattern: a huge wave of energy in radio wavelengths that lasted less than a second – and then the burst was gone, never to repeat. We initially suspected that FRBs could be hardware failures in our detectors, but over time, the repetition of the bursts convinced us that they were real.
Since then, we have identified sources of repeated bursts and linked the FRBs to a source that produces energy outside the radio range. This ultimately helped us point the finger at a single source: magnetars, or neutron stars with extremely intense magnetic fields.
Now reality has gone and that beautiful and simple explanation has thrown a wrench. A new repeating source of FRBs has been identified and is located in a location where we would not expect magnetars. This does not mean that the source is not of a magnetar, but we have to resort to some unusual explanations for its formation.
spinning neutrons
A magnetar is a form of a neutron star, left over from 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 is only about 20 kilometers across. That compact object inherits all of its rotational energy from its parent star, causing it to spin at a high speed, often aided by the addition of matter invading from its surroundings.
In many cases, this rapid rotation results in pulsars, neutron stars with radiation sources that appear to blink rapidly with the star’s rotation. In some others, the neutron star ends up with an intense magnetic field, turning it into a magnetar. A magnetar’s intense magnetic field lines are spun by its rotation, often creating high-energy interactions with its environment.
But these high-energy phenomena usually don’t last long, at least in astronomical terms. All of these energetic interactions with the environment cause the neutron star to lose energy, slowing its rotation and decreasing the intensity of the light it produces. Magnetars, for example, are typically thought to have a lifespan of just 10,000 years before disappearing into a quieter existence.
In addition, the supernovas that form magnetars occur in relatively young stars, usually only a few million years old.
This combination – an early star death and a short magnetar lifetime – means we expect magnetars only in regions with an abundance of young stars. Older stellar populations should have seen magnetars form and die billions of years earlier.
Where did that come from?
The new work, conducted by a large international team, involved the follow-up to the discovery of another FRB repeating resource, called FRB 20200120E. To determine 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 around the world. The team managed to point enough of those telescopes at the repeating source to image five separate FRBs.
The way reconstructing data from these different telescopes works, a single burst won’t give us a precise location. Instead, a range of possible locations can be identified. By combining the locations 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 in the nearby galaxy M81. Based on the remaining uncertainty about the location of FRB 20200120E and the frequency of globular clusters within M81, the research team estimates that the probability that FRB 20200120E is not located in this globular cluster is only about 1 in 10,000.
Searching that location revealed no lasting source of radio signals. Also, no high-energy sources were found, based on searches with X-ray and gamma-ray telescopes. So there is no obvious high-energy object there.
What is old is also new?
This location is strange. Globular clusters are most notable because they are made up of populations of old stars. It is unlikely that neutron star-forming supernovas have existed in it for billions of years. So that should probably rule out the presence of a magnetar, right?
Not quite. A handful of mechanisms could produce a magnetar without a supernova or long after a supernova has occurred. These mechanisms usually depend 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 different combinations of white dwarfs and neutron stars can merge, creating a neutron star as well. Finally, we know that a normal companion can “spin” a previously silent neutron star by feeding it matter.
Each of these processes could potentially produce a magnetar within a population of ancient stars. Which process – if any of them – actually took place on FRB 20200120E can be difficult to ascertain, given the apparent absence of any non-burst activity on the site.
In any case, the finding suggests that, if magnetars were the source of all FRBs, we might expect to see them in a much wider range of environments than had been predicted before this discovery. And maybe we don’t want to rule out considering non-magnetic sources just yet.
Nature, 2022. DOI: 10.1038/s41586-021-04354-w (About DOIs).
