Gamma rays are a broad category of high-energy photons, including everything with more energy than an X-ray.
That said, the list is larger than one, which means detecting gamma rays doesn’t mean we know what event produced them.
At lower energies, they can be produced in the areas around black holes and by neutron stars.
Supernovae can also produce a sudden burst of gamma rays, as can the merger of compact objects like neutron stars.
These are neutron stars that, at least temporarily, have extreme magnetic fields—over 1012 times stronger than the Sun’s magnetic field.
The directional data from Integral placed GRB 231115A right on top of a nearby galaxy, M82, which is also known as the Cigar Galaxy.
So, it looks like a magnetar giant flare, and the potential explanations for a brief burst of gamma radiation don’t really work for GRB 231115A.
The results could help us develop a template for distinguishing when we’re looking at a giant flare instead of alternative sources of gamma rays.
All high-energy photons with energy greater than that of an X-ray fall into the broad category of gamma rays. They are frequently produced by processes such as radioactive decay, but very few astronomical events generate them in large enough quantities to be detected when radiation from another galaxy is coming from that galaxy.
However, since the list is greater than one, we cannot determine the exact event that produced gamma rays just by detecting their presence. They can be created at lower energies by neutron stars and the environments surrounding black holes. Along with supernovae, mergers of compact objects like neutron stars can also result in an abrupt burst of gamma rays.
Magnetars are another type of object. These are neutron stars with extremely strong magnetic fields—more than 1012 times stronger than the magnetic field of the Sun—at least momentarily. Flares, including giant flares that release massive amounts of energy, including gamma rays, can occur on magnetars. The only known magnetar giant bursts have occurred in our own galaxy or its satellites, and these can be challenging to differentiate from gamma-ray bursts produced by the merger of compact objects. It seems up until now.
What did that consist of?
Among others, the Integral gamma-ray observatory of ESA observed the burst in question in November 2023. At some wavelengths, the duration of GRB 231115A was only approximately 50 milliseconds. This brief burst is comparable to gamma-ray bursts that are predicted to occur when neutron stars merge, although longer bursts can be produced when black holes form during supernovae.
The nearby galaxy M82, also referred to as the Cigar Galaxy, is directly above GRB 231115A according to the directional data obtained from Integral. M82 is classified as a starburst galaxy, indicating that it is rapidly forming stars. Interactions with its neighbors are probably what caused the burst. When compared to the Milky Way, the galaxy is star-forming more than ten times faster overall. This indicates a high number of supernovae as well as a sizable population of young neutron stars, some of which will develop into magnetars.
It remains possible, however, that M82 was coincidentally positioned in front of a gamma-ray burst from a far-off event. The most plausible explanation for the gamma rays, however, is something occurring inside the galaxy, as demonstrated by the researchers’ two distinct techniques, which rule out this possibility.
Although the estimated total energy of the burst is significantly less than what we would anticipate from those events, it is still possible that a gamma-ray burst is occurring within M82. Other wavelengths should also reveal the presence of a supernova, but none was seen here (and they usually produce longer bursts anyway). Our gravitational wave observatories would have been able to identify a signal from an alternative source, such as the fusion of two compact objects like neutron stars, but at this time there was none to be found. X-ray sources are also commonly left behind by these events, though M82 does not appear to have any new sources.
Thus, the observed phenomenon appears to be a giant flare of a magnetar, and the scenarios that could account for a transient spike in gamma rays are not very plausible for GRB 231115A.
searching for additional information.
The precise process through which magnetars emit gamma rays is still not fully understood. The extreme forces produced by the incredibly strong magnetic field are assumed to be responsible for the neutron star’s crust rearranging. Earth’s magnetic field is less than one gauss, and giant flares are thought to require magnetic field strengths of at least 1015 gauss.
The researchers estimate that 1045 ergs, or about 1022 megatons of TNT, of energy were released in total, assuming that the event directed radiation away from Earth and instead sent it off in all directions. Therefore, even though it’s not as energetic as mergers of neutron stars, it’s still a very energetic event.
Nevertheless, beyond the three cases in our local neighborhood that are clearly linked to magnetars, we likely need more to fully comprehend them. For astronomers, therefore, a major victory would be being able to reliably determine when these events occur in more distant galaxies. The findings may aid in the creation of a template that will allow us to differentiate between massive flares and other gamma-ray sources.
In addition, the researchers point out that this is the second possible giant flare linked to M82, and as was previously mentioned, starburst galaxies should be comparatively magnetar-rich. We may need to concentrate our searches in order to increase the frequency of our observations of it and other galaxies that are similar.
