We may have detected the first magnetar flare outside our galaxy

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

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

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

What was that?

The burst in question was detected 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, this short burst is similar to those expected to be seen when neutron stars merge.

Integral’s directional data placed GRB 231115A just above a nearby galaxy, M82, also known as the Cigar Galaxy. M82 is what is called a starburst galaxy, meaning it is forming stars at a rapid rate, and the burst was 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 a lot of supernovae, but it also means a large population of young neutron stars, some of which will form magnetars.

That doesn’t rule out the possibility that M82 was sitting in front of a gamma-ray burst from a distant event. However, researchers use two different methods to show that this is quite unlikely, leaving something happening inside the galaxy as the most likely source of gamma rays.

It could still be a gamma-ray burst within M82, except 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 usually produce longer explosions 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 detected at the time. These events also often leave behind X-ray sources, but no new sources are seen in M82.

So, it looks like a giant magnetar flare, and none of the possible explanations for a brief burst of gamma radiation really work for GRB 231115A.

Searching for more

The exact mechanism by which magnetars produce gamma rays is not fully resolved. It is believed to involve the reorganization of the neutron star’s crust, forced by the intense forces generated by the astonishingly intense magnetic field. Giant flares are thought to require magnetic fields of at least 10^fifteen gauss; The Earth’s magnetic field is less than one gauss.

Assuming that the event sent radiation in all directions rather than directing it toward Earth, the researchers estimate that the total energy released was 10^{Four. Five} ergs, which translates to approximately 10²² megatons of TNT. So while it is less energetic than neutron star mergers, it is still an impressively energetic event.

However, to understand them better we probably need more than the three cases in our immediate vicinity that are obviously associated with magnetars. Therefore, being able to consistently identify when these events occur in more distant galaxies would be a major victory for astronomers. The results could help us develop a template for distinguishing when we are looking at a giant flare rather than alternative sources of gamma rays.

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

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