2024-07-19 04:00:10
By François Gonzalez and Bertrand Cordier
Gamma-ray bursts are the brightest events ever observed in the Universe since the Big Bang. They are transient phenomena, that is to say very succinct, characterized by an intense flash of gamma rays. They are unpredictable, ephemeral and of still uncertain origin. What physics do they hide? Are they the messengers of the first stars of the Universe? Do they allow us to probe the past? The main objective of the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor), launched on June 22, is to study gamma-ray bursts to reveal their mysteries.
Oliver Sattler/CNES
Why observe gamma-ray bursts?
Manifestation of a gigantic explosion, the power released by gamma-ray bursts is considerable, equivalent to more than a billion billion suns. Scrutinized from every angle, they will no longer be seen as mysterious objects, but rather as formidable revealers of the unknown. Witness to a bygone past, they will give indications as to their original environment and the conditions of their formation, and will allow us to better understand theastrophysics from the extreme.
We now know that gamma-ray bursts occur in very distant galaxies, so distant that some of them are listed as the most distant objects measured today. However, given the finite speed of light, looking far away is like going back in time and thus observing far into the past! Like ephemeral lighthouses of the cosmos, gamma-ray bursts therefore make it possible to probe the Universe at different times in its history and to better understand how galaxies were formed over time. They also give us indications on the composition of the Universe throughout its history.
In addition, research teams from around the world will be able to observe transient cosmic phenomena, i.e. ephemeral objects whose brightness varies over time, such as supernovae or phenomena that cause gravitational waves. This strong involvement in the observation of transient phenomena will make the SVOM mission an essential partner for the entire community. scientist internationale.
Gamma-ray bursts, from laboratories to extreme conditions
The burst’s energy output, the speed of the ejected particles and the successive collisions with the surrounding environment are all elements that allow us to see gamma-ray bursts as laboratories offering us extreme conditions that are impossible to reproduce on Earth. The energy of the particles accelerated by the burst is up to a million times greater than what the most powerful terrestrial machines such as the Large Hadron Collider (LHC) du CERN can produce. Extremely massive stars (several hundred times the mass of the Sun) are, according to several scenarios, the cause of long bursts lasting up to several tens of minutes, a consequence of the collapse of these very massive stars. Studying these long bursts might help us better understand the first generation of stars in the Universe and how they formed in these distant times.
Conversely, short bursts (less than two seconds) might be associated with gravitational wave emissions. coalescence of two compact stars (neutron star and/or black hole) is a likely scenario to explain this phenomenon. However, the phases of the merger or the product of the coalescence (probably a black hole) are still poorly understood. Combined detections gamma ray burst – gravitational waves, as during the merger of two neutron stars observed on August 17, 2017, will consolidate the short burst scenario.
How do gamma-ray bursts form?
The majority of gamma-ray bursts recorded today seem to appear during the collapse of a star at the end of its life, massive enough to provide the energy necessary for the ejection of matter at very high speed. It is this matter, when it propagates in the surrounding environment, which will allow the transformation of this energy into gamma radiation. At the time of the explosion, there are several stages explaining the appearance of the burst, according to the so-called “fireball” model.
Illustration of the fireball model.
NASA’s Goddard Space Flight Center/Wikimedia
As it collapses, the star produces jets of matter, mainly electrons, ejected in jerks in a particular direction. This forms successive layers of matter that are expelled at different speeds but all of very large magnitude, close to that of light. These layers end up colliding, the fastest catching up with the slowest. These shocks will suddenly generate gamma rays. This is what is called prompt emission. This matter also interacts with the surrounding environment of the star. This gives rise to less intense, less energetic radiation that spreads out over time. It includes X-rays, visible light and radio waves. This is the followingglow emission. Afterglow emission lasts longer than prompt emission. It gradually decreases over a few hours, days or even months. This allows observation programs to be carried out with telescopes on the ground or in space, provided that a sufficiently precise position of the burst is available. In addition, to perceive a gamma-ray burst, the jets of matter emitted by the star must be well aligned in the direction of the Earth. The information provided by the followingglow is essential for a better understanding of the phenomenon triggering the bursts and the conditions prevailing in theenvironment stars that produce them.
The objectives of the SVOM mission
The SVOM mission was designed to answer these questions. But observing gamma-ray bursts is not a simple matter! The scientific study of gamma-ray bursts must indeed meet multiple constraints, each more complex than the last. First, the flash of gamma light must be identified. It can appear at any time and anywhere in the sky and only lasts a few seconds. As soon as it appears, a real race once morest time begins.
What’s more, gamma radiation is blocked by the Earth’s atmosphere! Detection must therefore take place from space. The instruments installed on the SVOM satellite are tasked with collecting data that will be transmitted to Earth in a minimum of time. The major challenge of the mission is to be able to determine the origin of the gamma-ray burst: where did it come from and when did it occur?
Only spectral analysis of light, that is, studying what wavelengths it is composed of, will make it possible to answer this last question, an analysis that will be carried out by large terrestrial telescopes from visible and infrared radiation that follows gamma rays. Coordinated observations at different wavelengths are therefore the key to understanding this astronomical phenomenon. The SVOM mission will have to provide a sufficiently precise location of the burst so that, from Earth, it is possible to observe it while the initial explosion loses energy and intensity.
Franco-Chinese cooperation
The SVOM mission is a Franco-Chinese mission launched on June 22, 2024 by the Chinese Long March 2C rocket from the launch base from Xichang, Sichuan province. It is the result of a collaboration between the two national space agencies, the China National Space Administration (CNSA) and the National Center for Space Studies (CNES). The satellite weighs a total of 950 kg for a charge utile of 450 kg. Placed on a low earth orbit At an altitude of 625 km, it will take only 96 minutes to orbit the Earth.
The mission has four main instruments, two of which are French and two of which are Chinese, and each of these instruments has a specific observation range. As for the French instruments, the telescope ECLAIRS is tasked with detecting and locating gamma-ray bursts in the low-energy X-ray and gamma-ray band (4 to 250 keV or kiloelectronvolts) while the telescope MXT (Microchannel X-ray Telescope) will observe gamma-ray bursts in the soft X-ray range (0.3 to 10 keV).
On the Chinese instruments side, the gamma ray burst detector GRM (Gamma Ray Burst Monitor) measures the spectrum of high-energy bursts (from 30 keV to 5000 keV) and the telescope VT (Visible Telescope) operates in the visible light domain to detect and observe the emission produced immediately following a gamma-ray burst.
Instruments on board the SVOM satellite.Shanghai Engineering Centre for Microsatellites (SECM), Provided by the author
Observations from space will be complemented by ground-based instruments including the wide-field camera Evacuate (Ground-based Wide Angle Camera) to study, in the visible domain, the prompt emission of a part of the detected bursts and robotic telescopes GFT (Ground Follow-up Telescope) to accurately measure the coordinates of the gamma-ray burst. SVOM is an ambitious scientific mission in the growing field of transient sky research. By deploying interconnected instruments on the ground and in space, and with scientists on call 24 hours a day, SVOM should enable significant advances in the understanding of gamma-ray bursts and the use of these objects to better study the young Universe and get closer to the very first stars.
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