We may have detected the first magnetic flare outside our galaxy

Gamma rays are a broad class of high-energy photons, including anything with greater energy than x-rays. While they often arise from processes such as radioactive decay, few astronomical events produce them in sufficient quantities that they can be detected when the radiation originates in another galaxy.

However, the list is larger than one, which means that the discovery of gamma rays does not mean that we know the event that led to their appearance. At low energies, they can be produced in the regions surrounding black holes and neutron stars. Supernovas can also produce a sudden burst of gamma rays, as can the merger of compact objects such as neutron stars.

Then there are magnetars. These are neutron stars that, at least temporarily, have intense magnetic fields of >10¹² Many times stronger than the sun's magnetic field. Magnetars can experience flares and even giant flares as they emit copious amounts of energy, including gamma rays. These explosions can be difficult to distinguish from gamma-ray bursts resulting from the merger of compact objects, so the only confirmed giant magnetar explosions have occurred in our galaxy or its satellites. So far it seems.

what was that

The explosion in question was monitored by the European Space Agency Integrated Gamma Ray Observatory, among others, in November 2023. GRB 231115A was short, 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.

Directional data from Integral GRB 231115A placed it directly above a nearby galaxy, M82, which is also known as the Cigar Galaxy. M82 is a so-called starburst galaxy, which means it is forming stars at a rapid clip, and the explosion is likely caused by interactions with its neighbors. Overall, the galaxy is forming stars at a rate more than 10 times the star formation rate of the Milky Way. This means a lot of supernovae, but it also means a lot of young neutron stars, some of which will form magnetars.

This does not rule out the possibility of M82 being present in front of a gamma-ray burst from a distant event. However, researchers are using two different methods to show that this is highly unlikely, making something occurring within the galaxy 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 explosion is much lower than we would expect from those events. Supernovas should also be detected at other wavelengths, but there was no sign of one (they usually produce longer explosions anyway). An alternative source, the merger of two compact objects such as neutron stars, could have been detected using gravitational wave observatories, but there was no clear signal at this time. These events often leave behind X-ray sources, but no new ones are visible in M82.

So, it looks like a giant magnetic flare, and possible explanations of a short gamma radiation burst don't really work for GRB 231115A.

Looking for more

The exact mechanism by which magnetars produce gamma rays has not been fully determined. This process is thought to involve a rearrangement of the neutron star's crust, imposed by the intense forces generated by the astonishingly intense 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 toward Earth, the researchers estimate that the total energy released was 10⁴⁵ ergs, which translates to approximately 10²² Megatons of TNT. So, although it is less active than a neutron star merger, it is still an impressively active event.

However, to understand them better, we probably need more than the three states in our immediate vicinity that are clearly associated with magnetars. So, being able to consistently determine when these events occur in distant galaxies would be a big win for astronomers. The results could help us develop a model to distinguish when we look at a giant flare rather than alternative sources of gamma rays.

The researchers also note that this is the second giant flare candidate associated with M82, and as mentioned above, stellar galaxies are expected to be relatively rich in magnetars. Focusing searches on them and similar galaxies may be just what we need to boost the pace of our observations.

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