The mystery of long gamma-ray bursts with kilonovas has undoubtedly been solved

More than 50 years ago, military satellites placed into orbit by the United States detected gamma-ray bursts, in English. Gamma ray bursts Or GRB. The mission of these satellites was to detect prohibited nuclear explosions inside or outside the atmosphere. But the scientists responsible for the satellites soon realized that these events were cosmic and not of human origin at all. Years later, their discovery was declassified, which would baffle the astrophysicist community.

In fact, the energy released was enormous, and not even understood until someone suggested recognizing that these gamma-ray bursts were not gamma radiation emissions according to some kind of light field, but according to focused jets. The energy released was much lower, although still enormous, but this time it was understood within the framework of known astrophysics.

Epic reveal of GW170817. For a fairly accurate French translation, click on the white rectangle at the bottom right. The English translation should appear next. Then click on the nut to the right of the rectangle, then on “Translations” and finally on “Translate automatically.” Select “French”. © Science versus cinema

Kilonova, gamma and gravity sources

We also realized that gamma-ray bursts can be divided into two categories: short, lasting less than two seconds, and long, often lasting about ten seconds. In the first case, these were probably collisions of neutron stars, resulting in what were later called kilonovas, explosions more powerful than novae but weaker than supernovae. We ended up validating this hypothesis by discovering the source of the gravitational wave GW170817.

These long explosions must have been caused by very massive stars in rapid rotation, which, by gravitationally collapsing, formed a black hole in their core, and then a black hole that accumulates matter and produces two jets as the star continues to rotate. Before it becomes a supernova it is sometimes referred to as a supernova. This is also called a model in English Collapses, Contraction of English terms Collapses (collapse) and a star (star), officially “exploded star” in French, although this name is rarely used in practice.

Things have become complicated since December 11, 2021 when the X-ray and gamma-ray detectors on board the Swift and Fermi satellites reported a GRB that lasted for about a minute and is therefore typical of a long gamma-ray burst and could be associated with a known galaxy. A billion light-years away from the Milky Way Galaxy.

However, observations can be made at different wavelengths such as infrared, for example, using a telescope Gemini North (Hawaii, USA) or with Hubble, clearly showed that GRB 211211A has properties that had hitherto been attributed to kilonovas, and thus to short gamma-ray bursts.

On December 11, 2021, NASA’s Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope detected a burst of high-energy light coming from the outskirts of a galaxy about a billion light-years away. This event shook scientists’ understanding of gamma-ray bursts (GRBs), the most powerful events in the universe. This explosion is called GRB 211211A. Several research groups have looked at observations collected by Swift, Fermi, the Hubble Space Telescope, and others. Some have suggested that the strangeness of this explosion could be explained by a neutron star merging with another massive object, such as a black hole. For a fairly accurate French translation, click on the white rectangle at the bottom right. The English translation should appear next. Then click on the nut to the right of the rectangle, then on “Translations” and finally on “Translate automatically.” Select “French”. © NASA Goddard Space Flight Center

Gottlieb ore He is a researcher Center for Computational Astrophysics (CCA) l Flatiron Institute in New York. He and colleagues have just published an article in Astrophysical Journal Letters A freely accessible version can be found at arXiv They announced that they had found an explanation for the paradoxical case of GRB 211211A and another occurring in 2023, GRB 230307A, which is similar.

The researchers performed sophisticated numerical simulations that specifically include the magnetohydrodynamics of plasma, which can form accretion disks in curved spacetime. They discovered that the observations could be interpreted in two ways, and in doing so they built a framework that unifies the description of many gamma-ray bursts, at least in theory.

A unified model of black holes accreting neutron star matter

In the first case, two neutron stars that collide in a normal kilonova scenario also form a black hole by merging. But what has changed is that the black hole is accumulating part of the matter left over from the collision, again forming an accretion disk and jets of matter that are sources of gamma rays.

The second case is a rare collision between a neutron star and a stellar black hole. But there we also get an accretion disk and jets of matter at the origin of a long gamma-ray burst.

This is what made Or Gottlieb say: Our findings, which link observations to fundamental physics, unify many unsolved mysteries in the field of gamma-ray bursts. For the first time, we can look at GRB observations and learn what happened before the black hole formed. If we see long GRBs like the one seen in 2022, we now know they come from a black hole with a massive disk. Knowing the existence of a massive disk, we can now determine the ratio of the masses of the two original objects because the ratio of their masses 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. »

Astrophysicists Northwestern University (USA) developed the first numerical simulation tracking the evolution of the jet during the merger of a black hole and a neutron star over large distances. © Uri Gottlieb, Danat Issa, Alexander Chekhovskoy

GRB discoveries are expected to double with assistance Vera C. Rubin Observatory Within a few years, which is why Gottlieb said in the conclusion of the Simons Foundation press release: “ As we obtain more observations of gamma-ray bursts at different pulsation periods, we will be better able to explore the central drivers that drive these extreme events. “.