Every now and then, astronomers will detect an odd kind of radio signal. So powerful it can outshine a galaxy, but lasting only milliseconds. They are known as fast radio bursts (FRBs). When they were first discovered a couple of decades ago, we had no idea what might cause them. We weren’t even sure if they were astronomical in origin. FRB’s were so localized and so short-lived, it was difficult to gather data on them. But with wide-field radio telescopes such as CHIME, we can now observe FRBs regularly and have a pretty good idea of their source: magnetars.
Magnetars are neutron stars with immensely powerful magnetic fields. Now that we can localize FRBs, we have been able to match a few of them to the region of a neutron star. While most FRBs occur in distant galaxies, in 2020 we observed one within the Milky Way. The magnetar source also happened to be a pulsar, and astronomers were able to show that the FRB [correlated with a glitch in the pulsar’s rotation,](https://briankoberlein.com/blog/power-of-magnetism/) thus confirming the source. So we are fairly certain that FRBs are caused by neutron stars, but we are still uncertain about the exact mechanism.
One popular idea is that fast radio bursts are caused by magnetic realignments. This is what drives flares on the sun. Over time, the sun’s magnetic field lines can get twisted up until they snap into realignment, releasing energy. If a similar effect occurs on magnetars, the resulting snap would be much faster and more powerful. One difficulty with this idea is that FRBs are so short-lived that they are almost too fast for magnetic field lines to realign. So astronomers keep looking for new ideas, and one recently proposed argues that they are caused by impact events.
Collisions have long been known as the source of high-energy events. For example, some supernovae are caused by the collisions of neutron stars. We also know that comets and asteroids occasionally impact the Sun, so we would expect similar impacts to occur on neutron stars. In this new work, the authors propose that FRBs are caused when an interstellar body collides with a neutron star. The impact would trigger a powerful electromagnetic burst. To support their argument, the authors looked at the distribution of FRBs arranged by duration. The timing of FRBs follows a distribution similar to the distribution of solar system bodies. Not only that, the duration of an FRB seems to match the hypothetical duration of an impact event based on an object’s size.
While the data does seem to support the idea of impact-based FRBs, the study doesn’t solve all the mysteries surrounding these powerful bursts. We know, for example, that some FRBs are repeaters, meaning they occur multiple times from the same source. Some studies have shown that repeating FRBs are quasi-periodic, which would be difficult to explain through random collisions. It’s possible that repeating and non-repeating FRBs are caused by different mechanisms, though the data is still inconclusive on that point.
