We may have seen the first magnetar burst outside our galaxy – Ars Technica

Gamma rays are a broad category of high-energy photons, including anything with more energy than an X-ray. While they are often created by processes such as radioactive decay, few astronomical events produce them in sufficient quantities to be are detected when the radiation originates in another galaxy.

That said, the list is bigger 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 regions around black holes and by neutron stars. Supernovae can also produce a sudden burst of gamma rays, as can compact objects such as neutron stars.

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

What was that?

The burst in question was observed by ESA’s Integral Gamma-ray Observatory, among others, in November 2023. GRB 231115A was short, lasting only about 50 milliseconds at several wavelengths. While longer bursts of gamma rays can be produced by the formation of black holes during supernovae, this short burst is similar to those expected to be seen when neutron stars merge.

Directional data from Integral placed GRB 231115A right on top of a nearby galaxy, M82, which is also known as the Cigar Galaxy. M82 is what’s called a starburst galaxy, meaning it’s forming stars at a rapid clip, with the explosion likely caused 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 sitting in front of a gamma-ray burst from a distant event. However, the researchers use two different methods to show that this is highly 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 those events. A supernova should also be detected at other wavelengths, but there was no sign of one (and they usually produce longer bursts anyway). An alternative source, the merger of two compact objects such as neutron stars, would be detectable using our gravitational wave observatories, but no signal was visible at this time. These events also often leave X-ray sources behind, but no new sources are visible in M82.

So it looks like a giant magnetar burst, and possible explanations for a brief gamma-ray burst don’t really work for GRB 231115A.

Looking for more

The exact mechanism by which magnetars produce gamma rays is not fully understood. It is thought to involve the rearrangement of the neutron star’s crust, forced by the intense forces created by the extremely intense magnetic field. Giant explosions are thought to require magnetic field strengths of at least 10¹⁵ gauss; The Earth’s magnetic field is less than one gauss.

Assuming that the event sent radiation in all directions instead of directing it toward Earth, the researchers estimate that the total energy released was 10⁴⁵ ergs, which translates to approximately 10²² megatons of TNT. So while it’s less energetic than a neutron star merger, it’s still an impressively energetic event.

However, to understand them better, we probably need more than the three cases in our immediate neighborhood that are definitely 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 results could help us develop a template for distinguishing when we’re seeing a giant flare instead of alternative gamma-ray sources.

The researchers also note that this is the second candidate giant outburst associated with M82 and, as mentioned above, starburst galaxies are expected to be relatively rich in magnetars. Focusing research 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 (About DOIs).