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What is a gamma-ray burst ?

Manifestation of a gigantic explosion, these flashes of light are considered to be the brightest and most energy-rich events since the Big Bang.

Un sursaut gamma s’accompagne d’un dégagement de matière sous forme de deux cônes opposés. https://www.eso.org/public/france/images/eso0917a/

A gamma-ray burst is accompanied by a release of material in the form of two opposite cones. Credit: ESO

These are rare events on the scale of a galaxy, yet it would be possible to observe on average ten per day.
The power that emerges is considerable, the equivalent of more than a billion billion suns. This makes them visible at very large distances, even beyond our galaxy. However, their detection is difficult and real technical prowess is needed to observe the random and unpredictable gamma-ray bursts.

A rush of gamma photons

These explosions are called « gamma-ray » in reference to the particles of very high energy initially released: gamma photons. Just like visible light, X-rays or radio waves, these are  electromagnetic waves.

Les rayons gamma (?) sont les ondes de plus petites longueurs d’onde (?) du spectre élecromagnétique. Le spectre de la lumière visible, à titre indicatif, est représenté par les couleurs selon la longueur d’onde croissante.

Gamma rays are the electromagnetic waves with the smallest wavelengths in the electromagnetic spectrum. The spectrum of visible light, as an indication, is represented by the colors.

The energy of a photon is measured in electron-volt (eV), a photon of visible light emits about 2eV. That of a gamma photon can reach several billions of electrons-volts!

Birth of a gamma ray burst

The explosion at the origin of the burst fulfills specific conditions. Today two scenarios explain the power and rapid variation of gamma ray bursts: the coalescence or fusion of two compact objects (neutron star or black hole) and the collapse of a very massive star.
The majority of gamma ray bursts recorded today indicate that they appear during the death of a very massive star. Indeed, under the scenario of the collapse of a star at the end its life, this one must be very massive to provide the energy required for the ejection of matter at very high speed. It is this material, when propagated in the surrounding environment, that will allow the transformation of this energy into gamma radiation.
At the moment of the explosion, we can distinguish several stages explaining the appearance of the burst, according to the so-called « fireball » model:

Illustration du modèle de la boule de feu. Crédits : NASA

Illustration of the fireball model. Credit : NASA

  1. The progenitor produces jets of material consisting essentially of packets of electrons, ejected in sporadically in a particular direction. These packets are expelled at different speeds but all close to the speed of light. These jets are therefore said to be ultra-relativistic.
  2. Very violent shocks take place when these packets of electrons come into contact with each other: it is the model of the internal shocks. The layers of material expelled at different speeds end up colliding, the fastest layers catching up with the slowest. These shock fronts will abruptly generate gamma rays. This is called the prompt emission.
  3. There are also external shocks where these same layers of matter interact later with the surrounding environment of the progenitor. This gives rise to less intense, less energetic radiation, which is spread out over time and composed of X-rays, visible light and radio waves. This is called the afterglow emission.

The conditions of appearance

The duration of the burst indicates two possible origins:
When the duration is less than 2 seconds, the burst is called a short burst. It would come from the coalescence of two massive and compact objects such as two neutron stars, or a neutron star and a black hole.
These two stars, in orbit, end up « falling » on each other as they lose energy by the emission of gravitational waves. From this ultimate encounter a new black hole is born.
In the case of long bursts, those lasting more than 2 seconds, they are produced at the end of a hypernova, a type of supernova. A hypernova is a star whose mass is greater than 20 times that of the sun and which undergoes gravitational collapse. A black hole is created abruptly causing shock waves that explode the rest of the star and pierce the stellar envelope: the outer layers are violently expelled. This is called the fireball model.
In both cases,  the newly-formed compact object (probably a black hole) grows swallowing  the matter in its immediate surroundings in a matter of seconds and forms a thick, rapidly rotating accretion disk arond itself. A part of the matter attracted by the gravitational force of the black hole is expelled in the form of two opposing jets along the axis of rotation of the disc following a physical mechanism still far from being understood.
This ejection at very high speed generates the shocks previously described and reveals the gamma-ray burst, as a result of internal shocks. In order to perceive its light, the observer must therefore be aligned with the axis of emission.

The afterglow emission

The afterglow emission of a gamma-ray burst is the phase which follows the prompt emission. According to the fireball model, it is due to shocks which during their expansion will sweep the environment surrounding the progenitor, generating radiation at all wavelengths. Its study thus makes it possible to know the nature of the environment of the progenitor. The remanent emission is not as brief as the prompt emission. It gradually decreases on a timescale of hours, days or months. This makes it possible to carry out observing programs with ground or space telescopes, provided that there is a sufficiently precise position of the burst, in particular in the X-rays and in visible light. The information provided by the afterglow emission is crucial for a better understanding of the explosive phenomenon and the environment of gamma-ray burst progenitors.

The fading image of the optical afterglow of GRB 030329, as seen on April 3 (four days after the GRB event) and May 1, 2003. The images were obtained with the FORS 1 and 2 multi-mode instruments at the 8.2-m VLT telescopes.

On the left, image obtained on April 3, 2003 of the afterglow emission in optical of the burst appeared on March 29, 2003. Right, a month later, the emission is always visible but weaker because it decreases gradually. Credit: ESO

Nomenclature

GRB for Gamma Ray Burst, followed by detection date, yymmdd.
Example: GRB 970508 corresponds to a burst detected on the 8th of May 1997.

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Auteur : CEA / Irfu & Jesse Palmerio