NASA, ESA and the Hubble Heritage Crew
Gamma rays are a broad class of high-energy photons, together with all the pieces with extra vitality than an X-ray. Whereas they’re typically created by processes like radioactive decay, few astronomical occasions produce them in adequate portions that they are often detected when the radiation originates in one other galaxy.
That mentioned, the checklist is bigger than one, which implies detecting gamma rays doesn’t suggest we all know what occasion produced them. At decrease energies, they are often produced within the areas round black holes and by neutron stars. Supernovae also can produce a sudden burst of gamma rays, as can the merger of compact objects like neutron stars.
After which there are magnetars. These are neutron stars that, at the least briefly, have excessive magnetic fields—over 1012 instances stronger than the Solar’s magnetic area. Magnetars can expertise flares and even large flares the place they ship out copious quantities of vitality, together with gamma rays. These will be tough to tell apart from gamma-ray bursts generated by the merger of compact objects, so the one confirmed magnetar large bursts have occurred in our personal galaxy or its satellites. Till now, apparently.
What was that?
The burst in query was noticed by the ESA’s Integral gamma-ray observatory, amongst others, in November 2023. GRB 231115A was quick, lasting solely about 50 milliseconds at some wavelengths. Whereas longer gamma-ray bursts will be produced by the formation of black holes throughout supernovae, this quick burst is much like these anticipated to be seen when neutron stars merge.
The directional information from Integral positioned GRB 231115A proper on high of a close-by galaxy, M82, which is often known as the Cigar Galaxy. M82 is what is named a starburst galaxy, which signifies that it is forming stars at a fast clip, with the burst prone to have been triggered by interactions with its neighbors. General, the galaxy is forming stars at a price greater than 10 instances that of the Milky Approach. Which means numerous supernovae, nevertheless it additionally means a big inhabitants of younger neutron stars, a few of which is able to kind magnetars.
That does not rule out the likelihood that M82 occurred to be sitting in entrance of a gamma-ray burst from a distant occasion. Nonetheless, the researchers use two totally different strategies to point out that that is fairly inconceivable, which leaves one thing occurring contained in the galaxy as being the most certainly supply of the gamma rays.
It may nonetheless be a gamma-ray burst occurring inside M82, besides the estimated whole vitality of the burst is far decrease than we might anticipate from these occasions. A supernova must also be detected at different wavelengths, however there was no signal of 1 (they usually sometimes produce longer bursts anyway). Another supply, the fusion of two compact objects corresponding to neutron stars, would have been detectable utilizing our gravitational wave observatories, however no sign was obvious right now. These occasions additionally often go away behind X-ray sources, however no new sources are seen in M82.
So, it seems like a magnetar large flare, and the potential explanations for a short burst of gamma radiation do not actually work for GRB 231115A.
In search of extra
The precise mechanism by which magnetars produce gamma rays is not totally labored out. It’s thought to contain the rearrangement of the crust of the neutron star, compelled by the extraordinary forces generated by the staggeringly intense magnetic area. Big flares are thought to require magnetic area strengths of at the least 1015 gauss; Earth’s magnetic area is lower than one gauss.
Assuming that the occasion despatched radiation off in all instructions fairly than directing it towards Earth, the researchers estimate that the entire vitality launched was 1045 ergs, which interprets to roughly 1022 megatons of TNT. So, whereas it is much less energetic than neutron star mergers, it is nonetheless an impressively energetic occasion.
To grasp them higher, nonetheless, we in all probability want greater than the three situations in our rapid neighborhood which can be clearly related to magnetars. So, with the ability to persistently establish when these occasions occur in additional distant galaxies could be a giant win for astronomers. The outcomes may assist us develop a template for distinguishing after we’re taking a look at a large flare as an alternative of other sources of gamma rays.
The researchers additionally notice that that is the second candidate large flare related to M82 and, as talked about above, starburst galaxies could be anticipated to be comparatively wealthy in magnetars. Focusing searches on it and related galaxies is perhaps what we have to enhance the frequency of our observations.
Nature, 2024. DOI: 10.1038/s41586-024-07285-4 (About DOIs).
