NASA, ESA, Hubble Heritage Team
Gamma rays are a broad category of high-energy photons that include anything higher in energy than X-rays. They are often produced by processes such as radioactive decay, but few astronomical events produce them in sufficient quantities to be detectable when the radiation originates from another galaxy.
However, the list is larger than one, so just because gamma rays are detected doesn’t mean we know what event caused them. At lower energies, they can be produced by regions around black holes or by neutron stars. Supernovae can also produce sudden bursts of gamma rays, as can mergers of compact objects such as neutron stars.
And then there’s the magnetar. These are neutron stars with extreme magnetic fields of more than 10, at least temporarily.12 It is many times stronger than the sun’s magnetic field. Magnetars can experience flares or giant flares that release large amounts of energy, including gamma rays. These can be difficult to distinguish from gamma-ray bursts produced by compact astronomical mergers, so the only confirmed magnetar giant bursts have occurred in our galaxy or its moons. Until now, apparently.
What?
The burst in question was discovered by ESA Integrated gamma-ray observatoryGRB 231115A was short, lasting only about 50 milliseconds at some wavelengths. This short burst is similar to the bursts expected to be seen when neutron stars merge, although longer gamma-ray bursts can be produced by the formation of black holes during supernova explosions.
Directivity data from Integral placed GRB 231115A directly above the nearby galaxy M82, also known as the Cigar Galaxy. M82 is a so-called starburst galaxy, meaning it is rapidly forming stars, meaning its outbursts were likely caused by interactions with neighboring galaxies. Overall, this galaxy is forming stars more than 10 times faster than the Milky Way. That means a lot of supernovae, but it also means a large population of young neutron stars that form magnetars.
That doesn’t exclude the possibility that M82 happened to be sitting in front of a gamma-ray burst from a distant event. However, the researchers used two different methods to show that this is highly unlikely, and that something happening within the galaxy is most likely the source of the gamma rays. is shown.
It could still be a gamma-ray burst occurring within M82, except the estimated total energy of the burst is much lower than expected from those phenomena. Supernovae should also be detected at other wavelengths, but there was no sign of them (and they usually produce longer bursts anyway). An alternative source, the merging of her two small objects, such as a neutron star, would have been detectable using gravitational wave observatories, but there was no obvious signal at this time. In these phenomena he often leaves behind an X-ray source, but in M82 no new sources can be seen.
So this looks like a giant magnetar flare, and potential explanations for short bursts of gamma rays don’t really apply to GRB 231115A.
Looking for more?
The exact mechanism by which magnetars produce gamma rays is not completely understood. This is thought to involve a rearrangement of the neutron star’s crust caused by powerful forces generated by an incredibly strong magnetic field.A giant flare is thought to require a magnetic field strength of at least 10.15 Gauss; Earth’s magnetic field is less than 1 Gauss.
Assuming that the event released radiation in all directions rather than pointing it at Earth, the researchers estimate that the total energy released was 10.45 ergs, or about 10twenty two Megaton TNT. So while it’s less energetic than a neutron star merger, it’s still an impressive energetic event.
But to understand them more deeply, we probably need more than just the three entities in our immediate vicinity that are clearly related to magnetars. Therefore, being able to consistently pinpoint when these events occur in more distant galaxies would be a major victory for astronomers. The results could help develop templates for distinguishing when we are looking at a giant flare rather than an alternative gamma-ray source.
The researchers also note that this is a second candidate for a giant flare associated with M82, and that, as mentioned above, starburst galaxies are expected to be relatively rich in magnetars. In order to increase observation frequency, it may be necessary to focus searches on this galaxy and similar galaxies.
Nature, 2024. DOI: 10.1038/s41586-024-07285-4 (About DOI).
