We may have spotted the first magnetar flare outside our galaxy

Gamma rays are a broad category of high-energy photons, encompassing anything with more energy than an the radiation originates in another galaxy.

That said, the list is longer than one, which means that detecting gamma rays doesn’t mean we know what event produced them. At lower energies they can be produced in areas around black holes and by neutron stars. Supernovae can also produce a sudden burst of gamma rays, as well as the merger of compact objects such as neutron stars.

And then there are magnetars. These are neutron stars that, at least temporarily, have extreme magnetic fields, greater than 10¹² times stronger than the Sun’s magnetic field. Magnetars can experience flares and even giant flares in which they emit copious amounts of energy, including gamma rays. These can be difficult to distinguish from gamma-ray bursts generated by the merger of compact objects, so the only confirmed giant magnetar bursts have occurred in our galaxy or its satellites. Until now, apparently.

What was that?

The burst in question was spotted by ESA’s Integral gamma-ray observatory, among others, in November 2023. GRB 231115A was brief, lasting only about 50 milliseconds at some wavelengths. While longer gamma-ray bursts can be produced by the formation of black holes during supernovae, these short bursts are similar to what should be observed when neutron stars merge.

Integral’s directional data places GRB 231115A directly above a nearby galaxy, M82, also known as the Cigar Galaxy. M82 is what’s called a starburst galaxy, meaning it is forming stars at a rapid rate, and it’s likely that the explosion was triggered by interactions with its neighbors. Overall, the galaxy is forming stars at a rate more than 10 times that of the Milky Way. That means lots of supernovae, but it also means a large population of young neutron stars, some of which will form magnetars.

This does not rule out the possibility that M82 was faced with a gamma-ray burst from a distant event. However, the researchers use two different methods to show that this is rather unlikely, which leaves something happening within the galaxy as the most likely source of the gamma rays.

It could still be a gamma-ray burst occurring within M82, except that the estimated total energy of the burst is much lower than we would expect from such events. A supernova should also be detected at other wavelengths, but there was no sign of one (and they typically produce longer bursts anyway). An alternative source, the merger of two compact objects such as neutron stars, would have been detectable using our gravitational wave observatories, but no signal was evident at the time. These events often leave behind X-ray sources, but no new sources are visible in M82.

So, it looks like a giant magnetar flare, and potential explanations for a brief burst of gamma radiation don’t really work for GRB 231115A.

Looking for more

The exact mechanism by which magnetars produce gamma rays has not been fully understood. It is thought to involve the reorganization of the neutron star’s crust, forced by the intense forces generated by the incredibly strong magnetic field. Giant flares are believed to require a magnetic field strength of at least 10¹⁵ Gauss; The Earth’s magnetic field is less than one gauss.

Assuming that the event sent radiation in all directions rather than directing it towards Earth, the researchers estimate that the total energy released was 10⁴⁵ ergs, which translates to about 10²² megatons of TNT. So even though it is less energetic than neutron star mergers, it is still an extraordinarily energetic event.

To understand them better, however, we probably need more than the three cases in our immediate vicinity that are obviously associated with magnetars. So, being able to consistently identify when these events occur in the most distant galaxies would be a huge win for astronomers. The findings could help us develop a model to distinguish when we are observing a giant flare rather than alternative sources of gamma rays.

The researchers also note that this is the second candidate for the giant flare associated with M82, and as previously mentioned, starburst galaxies are expected to be relatively rich in magnetars. Focusing searches on it and similar galaxies may be what we need to increase the frequency of our observations.

Nature, 2024. DOI: 10.1038/s41586-024-07285-4 (DOI information).