An exotic long gamma-ray burst, one of the brightest known cosmic cataclysms, is believed to be the result of the collision of two compact stars initially migrating around a supermassive black hole, such as two neutron stars in the surrounding accretion disk. This black hole. This is the scenario proposed to explain the enigmatic GRB 191019A.
Gamma-ray bursts (GRBs) remained unknown to the astrophysicist community for years after their discovery. It was only after military secrecy was lifted in 1973 over sightings of the US Vela family of satellites that the same community began to wonder about their existence. She therefore understood that the gamma-ray bursts detected in July 1967 by these satellites, which originally served to verify the application of the treaties banning nuclear weapons tests in the atmosphere, could not have come from the astrophysical events known at the time, such as solar flares or supernovae and classical novae.
The GRBs were so extraordinarily bright that they seemed to defy known physics, until it was realized that most of the energy released could be in the form of focused beams rather than an omnidirectional explosion. We still got spectacular numbers as it is about a thousand times the mass of Earth converted mainly to gamma rays but also to X and visible by a GRB.
We know they are of two main types: short gamma-ray bursts lasting two seconds at most, and long gamma-ray bursts lasting only a few hours. It wasn’t until the 1990s and 2000s that we began to really understand the theory behind these cosmic catastrophes and to be able to pinpoint them relatively accurately on the celestial canopy.
A very nice artist’s impression of the explosion of a hypernova with the formation of a black hole in the host star. These synthetic images illustrate the hypernova model that is believed to be responsible for most of the long gamma-ray bursts. Before a very massive star explodes, a black hole forms in the place of its heart, which then swallows the rest of the star. As an accretion disk also forms from streams of particles, we see these emerging from the star’s surface and propagating through the interstellar medium, creating a shock wave. This is where gamma photons are emitted. For a reasonably accurate French translation, click the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose French. © Desy, science communication laboratory
A kilonova mimicking a hypernova?
Brief gamma-ray bursts are thought to result from the collision of two neutron stars in a binary star system due to the loss of energy in the form of gravitational waves. The latest collision resulted in what is known as a kilonova, a scenario demonstrated a few years ago with the rise of gravitational astronomy by Ligo and Virgo, leading to the discovery in 2017 of GW source 170817 associated with GRB 170817A, a short gamma-ray burst originating from both the Fermi and Integral satellites.
Long gamma-ray bursts fall under the hypernovae scenario explained in the video above. In fact, the boundary between the two GRB types is a bit blurred and events have been discovered which, if at first glance they fell under one of the scenarios, could actually be better explained by the other, according to a recent article elsewhere shows published in the famous journal Nature Astronomy and a version of it is also available on arXiv.
In the Nature article, astrophysicists examined the case of GRB 191019A, a gamma-ray burst that lasted about a minute and was detected by NASA’s Neil Gehrel’s Swift Observatory satellite on October 19, 2019. Located at the heart of an ancient galaxy, it could also be observed from the ground, but its visible counterpart with the eight-meter telescope at Gemini South Observatory in Chile.
Since no traces of supernovae were detected at that time, and even fewer hypernovae, astrophysicists were forced to consider a different scenario to explain GRB 191019A, taking into account telescopic data that places it in a very special environment, namely a sphere of about 100 light-years across, surrounding a supermassive black hole. The star density there is enormous, up to about a million stars in a volume a few light-years across. As a reminder, the average distance between two stars in the disk of the Milky Way is about 4 light-years.
A new scenario for gravitational wave emissions
The researchers’ calculations therefore suggest that explaining GRB 191019A requires collisions between compact stars such as white dwarfs, neutron stars and black holes, possibly in the gas-rich accretion disk of the central supermassive black hole.
However, these were not collisions between celestial bodies originally part of a binary system, but rather the analogue of collisions between the molecules of a gas moving in all directions in a given volume.
“These new results show that stars can die in some of the densest regions of the Universe where they can be collided. “It is exciting to understand how stars die and to answer other questions, such as what unexpected sources might be generating gravitational waves that we might detect on Earth,” said NOIRLab (National Optical-Infrared Astronomy Research Laboratory, State of the USA) the NSF Center for Nocturnal Ground-Based Optical Astronomy) Andrew Levan, astronomer at Radboud University in the Netherlands and lead author of the article in the journal Nature Astronomy.