We may have discovered the first magnetar flare outside the Milky Way

Gamma rays are a broad class of high-energy photons that include all photons with higher energies than X-rays. While they are often produced by processes such as radioactive decay, few astronomical events produce enough of them that the radiation can be detected when originating in another galaxy.

That said, the list is bigger 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 region around black holes and in neutron stars. Supernovae can also produce sudden bursts of gamma rays, as can mergers of dense objects such as neutron stars.

And then there are magnetars.These neutron stars have, at least temporarily, extreme magnetic fields exceeding magnitude 10¹² Several times stronger than the solar magnetic field. Magnetars experience flares, even giant ones, which emit large amounts of energy, including gamma rays. These can be difficult to distinguish from gamma-ray bursts produced by mergers of compact objects, so the only confirmed magnetar giant bursts occur in our own galaxy or its satellites. So far, obviously.

what is that?

In November 2023, the European Space Agency’s Integral Gamma-ray Observatory and others discovered the burst in question. While the formation of a black hole during a supernova can produce longer gamma-ray bursts, this brief burst is similar to the gamma-ray bursts expected when neutron stars merge.

Orientation data from Integral places GRB 231115A on top of the nearby galaxy M82, also known as the Cigar Galaxy. M82 is a so-called starburst galaxy, meaning it is rapidly forming stars, and the bursts may be triggered by interactions with its neighbors. Overall, the galaxy forms stars more than 10 times faster than the Milky Way. That 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 that M82 happened to be in front of a gamma-ray burst produced by a distant event. However, the researchers used two different methods to show that this is quite unlikely, making what is happening inside the galaxy the most likely source of gamma rays.

It’s still possible that it was a gamma-ray burst occurring within M82, except that the estimated total energy of the burst was much lower than we would expect from these events. Supernovae should also be detected at other wavelengths, but there are no signs of them (and they usually produce longer bursts). Another source, the fusion of two compact objects (such as neutron stars), can be detected using our gravitational wave observatories, but at this time there is no obvious signal. These events also often leave behind X-ray sources, but no new sources are visible in M82.

Therefore, it looks like a magnetar giant flare, and any potential explanation for the brief burst of gamma radiation does not apply to GRB 231115A.

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The exact mechanism by which magnetars produce gamma rays is not fully understood. It’s thought that this involves a rearrangement of the neutron star’s outer shell, forced by the powerful forces created by a shockingly strong magnetic field.Giant flares are thought to require a magnetic field strength of at least 10¹⁵ Gaussian; Earth’s magnetic field is less than one Gaussian.

Assuming the event sent radiation in all directions, rather than directing it toward Earth, the researchers estimate the total energy released was 10⁴⁵ ergs, approximately equivalent to 10^{twenty two} Million tons of TNT. So while it’s not as energetic as a neutron star merger, it’s still an impressively energetic event.

To understand them better, however, we may need more than just three apparently magnetar-related instances in our neighborhood. So being able to consistently determine when these events occur in more distant galaxies would be a huge win for astronomers. These results could help us develop a template to distinguish when we see giant flares rather than alternative sources of gamma rays.

The researchers also note that this is the second candidate giant flare associated with M82, as starburst galaxies are expected to be relatively rich in magnetars, as mentioned above. A focused search for it and similar galaxies may be what we need to increase the frequency of our observations.

Nature, 2024.