(Paris) Flashes of light coming from very far away, with such intense energy that they can cross billions of light years: gamma-ray bursts are valuable witnesses to the early ages of the Universe, but astronomers are struggling to observe them as they are so fleeting.
The Franco-Chinese SVOM (Space-based multi-band astronomical Variable Objects Monitor) mission is preparing to explore these extreme phenomena of the cosmos, still mysterious more than 50 years after their discovery.
The 930-kilo satellite and its four onboard instruments (two Chinese, two French) will take off on Saturday aboard a Chinese Long March 2-C rocket from the Xichang space base (southwest China).
Chen Lan, specialist in the Chinese space program, praised to AFP the “political significance” of this mission, “in a rather dark period in relations between China and the West, which shows that scientific cooperation can continue despite difficulties “.
“The investigation started in the middle of the Cold War,” says Bertrand Cordier, scientific manager of the French contribution to SVOM.
It was 1967 when American satellites intended to monitor nuclear tests observed a brief flash emitting gamma rays – the most energetic waves in the electromagnetic spectrum, which are produced by the decay of radioactive elements.
At the beginning of the 1970s, the file entered the scientific field and “since then, we have tried to understand the origin of these phenomena”, explained this researcher at the CEA (Atomic Energy Commission), during a presentation of the French instruments from SVOM to the press.
Several space missions, including the American Swift telescope, have made it possible to identify these unpredictable events. First there is the emission of a so-called rapid flash of light, in gamma rays, lasting between a fraction of a second and a few tens of seconds.
What follows is an “afterglow” emission of X-rays, which can last a few hours, and which “crosses the entire Universe to reach us”, details Susanna Vergani, CNRS research director at the Paris-PSL Observatory .
The short flashes appear to come from the merger of compact objects, either two neutron stars or a neutron star with a black hole.
Long flashes would originate from the explosion of the very first massive stars, much larger than the Sun: a specific population of stars born at the beginning of the cosmos, which die violently after a short life.
They are therefore valuable tools for “probing the distant Universe”, in particular the little-known chemical process by which the first stars appeared, explains Susanna Vergani.
These bursts also have the advantage of bearing the imprint of the gas clouds that they have passed through during the different ages of the Universe, all markers of its history.
The Milky Way being too old to give birth to them, the probability of such a gamma-ray burst hitting us “is extremely low”, notes Bertrand Cordier, adding that the earth’s atmosphere would act as a screen if necessary.
Today the main difficulty consists of locating these phenomena. The satellite’s French ECLAIRs instrument is therefore equipped with a coded mask, a metal plate pierced with holes which, when illuminated by a gamma ray source, allows its direction to be reconstructed.
The extreme brevity of these phenomena will put scientists in a race against time to collect information in time.
As soon as SVOM detects a burst, it will send an alert to a team of scientists on call 24 hours a day. In less than five minutes, they will have to trigger an array of telescopes on the ground which will align themselves precisely in the axis of the source of the burst, for more in-depth observations.
