Gamma ray bursts are bursts of gamma photons which probably occur during the collision of a neutron star and a black hole but also when a massive star eventually ran out of fuel and collapsed to form a black hole.

This very particular radiation is located at the end of the electromagnetic spectrum which corresponds to the high frequencies. In fact, the gamma ray frequency range is so vast and so poorly understood that it has no well-defined upper limit.

They are the most violent, spectacular and luminous explosive phenomena in the Universe. This phenomenon is generally brief and lasts from a few seconds to a few minutes. However, on several occasions, gamma-ray bursts of several hours have been recorded, these would correspond to the collapse of a massive and compact star, about 25 times more massive than our star, the Sun.

During this phenomenon, colossal amounts of energy and matter are ejected, at a speed close to that of light. In just a few seconds, a typical burst releases as much energy as the Sun will produce in its ten billion years of existence.

Speeding away from the star, these jets collide with the previously emitted gases, considerably raising their temperature. The resulting emissions constitute what is called afterglow. The persistent emission, no doubt generated by the multiple collisions between the ejected matter and the surrounding gas, then gradually fades.

This phenomenon is very common and the Swift and Fermi space observatories detect on average one gamma-ray burst per day! Fortunately, they are almost all too far away and do not aim directly at Earth… A map of gamma-ray bursts updated in real time is available on the website of the University of Sonoma.

Currently, more than 150 known objects emit gamma radiation.

The most powerful gamma-ray bursts ever observed

In mid-November 2019, two gamma-ray bursts broke energy records never detected until then, confirming that it was possible for these gamma emissions to reach energy levels at least 1,000 billion times higher than that of visible light!

These gamma-ray bursts suggest that the initial explosion generated the formation of a plasma jet which, when it meets the interstellar medium, slows down and creates a shock wave which then acts as a “cosmic particle accelerator”.

The threat of the “death star” WR104

Discovered in 1998, WR104 is a giant end-of-life star of the Wolf-Rayet type located 8,000 light years from us, in the constellation Sagittarius. It is nicknamed the “death star” because her death could well lead to that of our planet.

“When this star comes to die, not only will it explode in a supernova, but its heart will collapse to form a black hole, flanked by two beams of gamma rays which will emanate from the poles, and one of which could point directly at us–a cataclysmic event which should occur within 500,000 years.

The consequences of a gamma-ray burst for the Earth

If a gamma-ray burst were aimed at Earth and came from the collapse of a star located less than 6000 light years from us, the consequences for Earth would be catastrophic. Indeed, one of the five massive extinctions of biodiversity that the Earth has known for 500 million years could be linked to such a phenomenon.

A gamma-ray burst hitting Earth would have serious consequences for life:

• Destruction of about 30% of the ozone layer for almost a decade. The UV rays of the Sun would then be twice as strong and carbonize the phytoplankton, at the base of the ocean food chain.
• Formation of large quantities of nitrogen oxide in the atmosphere which would then be tinged with yellow/orange comparable to that an urban smog, resulting in acid rain.
• Significant decrease in photosynthesis and collapse of around 60% of food production, resulting in chaos in human societies.

Scientists estimate the return period of such a cataclysm to be around 300 million years. The last one dates from 440 million years ago, causing the massive extinction of the Ordovician according to a study conducted by Dr. Melott, Dr. Thomas and their teams.

Their hypothesis explains why certain species had disappeared before the glaciation of this period and why the extinction mainly affected plankton and the populations of the surface marine layers.

2008: a burst of gamma rays directly targeted the Earth

In early 2008, telescopes around the world observed the brightest explosion to date. An international team of astronomers has revealed that it is in fact a burst of gamma rays coming from a galaxy located halfway between the limits of the observable universe, and resulting from a powerful jet of matter, sent directly to Earth.

This exceptional event offered astronomers an unprecedented vision of a gamma-ray burst, and the observations made after the explosion revolutionized our understanding of the phenomenon.

It all started on the morning of March 19, 2008. In Chile, the TORTORA telescope of the European Southern Observatory, as well as the Polish “Pi of the Sky” telescope, detected a very bright flash in the Boötes constellation. Far above the ground, NASA’s SWIFT satellite detected a gamma-ray burst from the same source. Within seconds, SWIFT would send an alert, and soon many telescopes around the world would turn to the event.

Barely an hour after the observation of the first flash, the VLT (Very Large Telescope) of the European Southern Observatory (ESO) revealed that the explosion had occurred at 7.5 billion light years, half the radius of the observable universe, far enough for it to remain without consequences for our planet.

Even at this distance, the explosion was so bright that it would have been visible to the naked eye, if someone had looked in its direction. If it had taken place in our own galaxy, its brightness would have matched the brightness of the sun.

The Cherenkov Telescope Array (CTA)

After 10 years of work, a new network of gamma radiation detectors, the Cherenkov Telescope Array (CTA) will be built from 2021. More than 1,500 scientists and engineers from 31 countries are working together to make this project a reality.

CTA will be the first network of terrestrial gamma ray observatories open to the international scientific community. The actual observations will be carried out by operators, the data and analysis tools will then be made available to the principal investigator in common data formats. After about a year, the data will be made public and accessible to communities in astrophysics, particle physics and other fields.

The Cherenkov Telescope Array (CTA) observatory will include 100 telescopes distributed over two sites in the northern (La Palma, Spain) and southern (Chile) hemispheres. Thanks to its ability to cover a huge range of photon energy from 20 gigaelectronvolts to 300 teraelectronvolts (TeV), the CTA will greatly exceed the performance and potential of current instruments. “In addition, its wider field of vision and its tenfold sensitivity will allow CTA to scan the sky hundreds of times faster than previous TeV telescopes,” said Federico Ferrini, CEO of CTAO gGmbH, the provisional legal entity in place to prepare for the implementation of CTA.

This observatory will open a new window on the most extreme events in the Universe.