We may have discovered the first magnetar flare outside our galaxy

Gamma rays are a broad category of high-energy photons that include photons with higher energy than X-rays. They are often produced by processes such as radioactive decay, but only a few astronomical events produce enough quantities to be detectable if the radiation comes from another galaxy. However, the list is larger than one, and just because gamma rays are detected does not mean we know which event caused the gamma rays. At lower energies, they can be produced near black holes or 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, at least temporarily, exceeding 1012 times 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 caused by compact object mergers. Therefore, the only confirmed giant magnetar eruptions have occurred in our own galaxy or its moons. Until now, apparently. What? The occurrence of the problem was discovered by ESA’s Integral Gamma-ray Observatory in November 2023, among other things. GRB 231115A was short, lasting only about 50 milliseconds at some wavelengths. Although longer gamma-ray bursts can be produced by black hole formation during supernova explosions, this short burst is similar to the bursts expected from neutron star mergers. Integral’s directional data placed GRB 231115A directly above the nearby galaxy M82, also known as the Cigar Galaxy. M82 is a so-called starburst galaxy. This means it is forming stars at a rapid pace, and outbursts can be caused by interactions with neighboring galaxies. Overall, this galaxy forms stars more than 10 times faster than the Milky Way. This means that there are not only large numbers of supernovae, but also large populations of young neutron stars that form magnetars. This does not exclude the possibility that M82 happened to be in front of a gamma-ray burst from a distant event. However, the researchers used two different methods to show that this is completely unlikely, meaning that something happening within the galaxy is most likely the source of the gamma rays. is showing. It is still likely that it is a gamma-ray burst within M82, except that the estimated total energy of the burst is much lower than expected based on these phenomena. Supernovae should have been detected at other wavelengths, but there was no evidence of that (and they usually produce longer bursts anyway). An alternative source, a merger of two small objects like a neutron star, would have been detectable by gravitational wave observatories, but no signal was detected at the time. In these phenomena he often leaves behind an X-ray source, but in M82 no new sources can be seen. So it looks like a giant magnetar burst, and possible explanations for short bursts of gamma rays don’t really hold true for GRB 231115A. Looking for more? The exact mechanism by which magnetars generate gamma rays is still not fully understood. This is thought to be due to the reorganization of the neutron star’s crust due to the powerful force generated by the surprisingly strong magnetic field. It is believed that a large flare requires a magnetic field strength of at least 1015 Gauss. Earth’s magnetic field is less than 1 Gauss. Assuming that the event emitted radiation in all directions and was not directed at Earth, the researchers estimated that the total energy released was 1045 ergs, which is equivalent to about 1022 megatons of TNT. are doing. So while it’s less energetic than a neutron star merger, it’s still an impressive energetic event. But to understand them better, we need more than just the three cases in our immediate vicinity that are probably 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 between observing giant flares and alternative gamma-ray sources. The researchers also note that this is the second giant burst potentially related to M82, and that, as mentioned above, starburst galaxies are expected to be relatively rich in magnetars. are doing. To increase the frequency of observations, it may be necessary to focus searches on these and similar galaxies.

source: http://dx.doi.org/10.1038/s41586-024-07285-4