INTEGRAL, a European satellite to detect the ‘vampire’ photons of astrophysics

With a tremendously evocative name for science fiction, gamma rays they have a reality, some physical properties, that far exceed what is imaginable. A variety of extreme, high-energy, violent, and short-lived events occur in the universe. Strangers.

the imagination of Bram Stoker in his dracula He gave vampires the property of not reflecting in mirrors. Gamma rays don’t either. They have a wavelength so small, or so short, that they directly pass through the space between the atoms in the mirrors we normally use in telescopes. For this reason, the detectors must be built in such a way that we can measure the products of the collision of these high-energy rays with the electrons. It is as if we did not have access to see what happens in the first shot of the pool table, but we do have access to see the balls that fall through the holes. By measuring its direction and speed, we reconstruct the original impact, that is, its location and energy.

This is how the gamma ray detectors on board the European satellite have been built INTEGRAL (International Gamma-ray Astrophysics Laboratory). When a high-energy photon collides with an electron from the crystals, built precisely so that there are collisions, that is, they are very tight, it measures the cascade of impacts with lower energy.

Gamma rays It is the name given to the highest energies of all the world. electromagnetic spectrum. It is logical, therefore, that they occur as part of the most catastrophic phenomena out there, in outer space, although they also occur in storms, in nuclear explosions or in radioactive processes. To find out what happens, for example, in positron annihilation processes in the galaxy, in a supernova explosion, in the collision of neutron stars or in the regions surrounding black holes, we have to look with detectors sensitive to high energies.

This is what the INTEGRAL satellite does, which just turned 20 years in an orbit which currently goes around the Earth three times in approximately 8 days. An impressive four tons of satellite that was launched into space by a Proton rocket from the Russian Federation, above the Van Allen radiation belts and whose closest distance to Earth reached 2,000 kilometers in height in 2020, but, since it is in an eccentric orbit, it can reach 150,000 km from our planet, almost halfway to the Moon.

High energies: photosynthesis depends on photons with less than 3 electron volts (eV) or vitamin D production uses 4 eV. The INTEGRAL satellite is sensitive to photons between 15,000 and 10 million eV

The INTEGRAL satellite in another artistic concept from the time of its launch, in the year 2000.THAT

And for what? In order to understand physics, the nature of phenomena such as gamma-ray bursts (GRBs) that they are the most energetic events in the entire universe after the Big Bang and that, being concentrated in beams, can emit in a few seconds all the energy that a star like the Sun emits in its entire life. As the most recent occurred just a month ago, GRB 221009Athe most violent ever detected, which despite having been generated at a whopping 2,000 million light-years from Earth, occurred so “close” in astronomical terms, that it was felt even in the upper part of the atmosphere.

One of the capabilities of INTEGRAL is, in fact, to discover and locate the position of these very high energy cosmic sources. In addition, it carries incorporated a small optical telescope “made in Spain” made in INTA and with principal investigator in the Astrobiology Center and that it has been two decades obtaining an image of the sky every 1-2 minutes.

The gamma ray flare occurs when a photon collides with an electron, resulting in a lower energetic electron and a more energetic photon.
The gamma ray flare occurs when a photon collides with an electron, resulting in a lower energetic electron and a more energetic photon.Institute of Astrophysics of Andalusia

As a reference of what type of energy we are talking about when we say that they are high, let us give as an example photosynthesisthat depends of photons with less than 3 electron volts (eV) or the production of vitamin D that uses photons that carry about 4 eV. Well then, INTEGRAL is sensitive to photons between 15,000 eV and 10 million eV.

Keeping our eyes on the high-energy sky teaches us what happens in X-ray binaries (those strange couples made up of a black hole and a star that donates material to it) or in the radioactive processes that manifest themselves in the explosion of supernovae. But perhaps the most spectacular thing has been the new window to knowledge that has been opened with the detection and localization of the short gamma-ray burst, the electromagnetic counterpart of the merger of two neutron stars, associated with the detection of gravitational waves GW170817.

The sky seems calm, but if we could see the gamma rays with our eyes it would be more similar to the lights of a town festival in summer or a nightclub late at night.

The economic cuts within the European Space Agency (ESA) endanger the continuity of INTEGRAL’s operations from 2023. Especially since high energy events are unpredictable, highly variable and rare. The extension of the operations would be fundamental to complement detections when the gravitational wave interferometers (LIGO, VIRGO, KAGRA) launch next year. With INTEGRAL we may be able to see for the first time the electromagnetic counterpart of a neutron star and a black hole.

If we could see the sky in gamma rays with our own eyes, it would be more like the lights of a summer village festival or a late-night disco: brilliant flashes of energy that flash on and off for seconds, or rarely minutes. Nothing similar to the serenity of a calm, starry and apparently immutable sky, that we see with the sensors that we have in our eyes. In order not to be fooled by this apparent tranquility, telescopes such as INTEGRAL are built: the European eyes to contemplate the universe in gamma rays.

Let’s hope that the ESA decides to continue with its eyes wide open when the new operations of the gravitational wave detectors begin next year.

Cosmic Void is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Perez Gonzalezresearcher at the Center for Astrobiology; Patricia Sanchez Blazquez, full professor at the Complutense University of Madrid (UCM); Y Eva Villaverresearcher at the Center for Astrobiology.

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INTEGRAL, a European satellite to detect the ‘vampire’ photons of astrophysics