2023-12-03 18:24:57
More than 50 years ago, military satellites put into orbit by the United States discovered gamma-ray bursts, in English gamma-ray bursts or GRB. These satellites had the mission of detecting prohibited nuclear explosions in or outside the atmosphere. But quickly, the scientists in charge of the satellites understood that these events were cosmic and not at all of human origin. Years later, their discovery was declassified, which would perplexe the astrophysicist community.
Indeed, the energy released was colossal, incomprehensible even until someone proposed to admit that these GRBs were not emissions of gamma radiation according to a sort of sphere of light, but according to focused jets . The energy released was much less although still gigantic, but this time understandable within the framework of known astrophysics.
The saga of the detection of GW170817. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © Science vs Cinema
Kilonovae, gamma and gravitational sources
We also realized that GRBs might be divided into two classes: short ones, lasting less than two seconds, and long ones, often lasting around ten seconds. In the first case, these were probably collisions of neutron stars, resulting in what were later called kilonovae, explosions stronger than novae but weaker than supernovae. We ended up validating this hypothesis with the detection of the gravitational wave source GW170817.
The long bursts must have been produced by very massive stars in rapid rotation which, by gravitationally collapsing, formed a black hole in their core, a black hole then accreting matter and producing two jets while the star continued to rotate. collapse onto the black hole before becoming a supernova sometimes referred to as a hypernova. This is what has also been called in English the model of collapsar, contraction of English terms collapse (collapse) and star (star), officially “imploding star” in French, although this name is hardly used in practice.
Things became complicated from December 11, 2021 when the X-ray and gamma detectors on board the Swift and Fermi satellites reported a GRB lasting almost a minute and therefore typical of a long gamma-ray burst and that the might be associated with a known galaxy a billion light years away from the Milky Way.
However, observations at different wavelengths such as infrared, for example, with a telescope Gemini North (Hawaii, United States) or with Hubble, clearly showed that GRB 211211A had characteristics that until now had been attributed to kilonovae and therefore to short gamma-ray bursts.
On December 11, 2021, NASA’s Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope detected a burst of high-energy light coming from the outskirts of a galaxy regarding 1 billion light years away. This event shook scientists’ understanding of gamma-ray bursts (GRBs), the most powerful events in the Universe. This burst is called GRB 211211A. Many research groups have looked at observations collected by Swift, Fermi, the Hubble Space Telescope and others. Some have suggested that the oddities of this explosion might be explained by the merger of a neutron star with another massive object, such as a black hole. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Automatically translate”. Choose “French”. © NASA’s Goddard Space Flight Center
Ore Gottlieb is a researcher at Center for Computational Astrophysics (CCA) you Flatiron Institute At New York. With colleagues he has just published an article in The Astrophysical Journal Letters a freely accessible version of which can be found on arXiv in which they announce having found an explanation for the paradoxical case of GRB 211211A and another occurring in 2023, GRB 230307A, which is similar.
The researchers carried out sophisticated numerical simulations involving in particular the magnetohydrodynamics of plasmas which can constitute accretion disks in curved space-time. They discovered that the observations might be accounted for in two ways and in doing so constructed a framework unifying the description of many gamma-ray bursts, in theory at least.
A unified model with black holes accreting neutron star matter
In the first case, the two neutron stars that collide in the scenario of an ordinary kilonova also form a black hole by merging. But what changes is that the black hole accretes part of the matter left by the collision, once more forming an accretion disk and jets of matter that are sources of gamma rays.
The second case is a variant with a rarer collision between a neutron star and a stellar black hole. But, there too, we obtain an accretion disk and jets of matter at the origin of a long gamma-ray burst.
This is what made Ore Gottlieb say: “ Our findings, which connect observations to the underlying physics, have unified many unsolved mysteries in the field of gamma-ray bursts. For the first time, we can look at GRB observations and know what happened before the black hole formed. If we see a long GRB like those seen in 2022, we now know that it comes from a black hole with a massive disk. And knowing that there is a massive disk, we can now determine the ratio of the masses of the two parent objects because their mass ratio is related to the properties of the disk. For example, the merger of neutron stars of unequal mass will inevitably produce a long-lived GRB. »
The astrophysicists of the Northwestern University (United States) have developed the first numerical simulation that follows the evolution of a jet during a black hole-neutron star merger over large distances. © Ore Gottlieb, Danat Issa, Alexander Tchekhovskoy
GRB discoveries are expected to multiply with the help of Vera C. Rubin Observatory in a few years, which makes Ore Gottlieb say at the conclusion of the Simons Foundation press release: “ As we obtain more observations of GRBs at different pulse durations, we will be better able to probe the central drivers that drive these extreme events. ».
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