A black hole is a region of space within which matter generates a gravitational field so strong that no material particle, including light, can escape. Black holes are formed by a process of gravitational collapse or inward collapse of a celestial body due to the effect of its own gravity. In the center matter converges towards a point called singularity. The spherical surface around a singularity that limits the zone where matter and energy can no longer escape is called an event horizon or event horizon, because events on one side of it cannot affect an observer on the other side. The black hole is dark, but the plasma around the event horizon heats up to billions of degrees, forming a bright accretion disk from which jets of photons can be emitted. Black holes can have a mass of up to ten billion times that of the Sun concentrated in a ridiculously small region of space and grow by swallowing stars or gas clouds in their surroundings or, when a collision between galaxies occurs, merging with other black holes. While the black hole is gobbling up matter, it emits up to 10% of it in the form of radiation. This is the most efficient method known in nature for converting mass into energy.

The possibility that there are bodies that generate a gravitational field so intense that not even light can escape from them was raised in 1783 by Michell (1724-1793), based on the concept of escape velocity. At that time, the corpuscular model of light, defended by Newton, still prevailed. A few years later, in 1796, Laplace (1749-1827), a follower of Newtonian mechanics, presented the same idea in the first two editions of his book Exposition du Systeme du Monde, although, as the wave theory of light gained ground, it was discarded in later editions. In this context, the concept had little progress due to the difficulty of establishing how light, which has no mass, can be affected by gravity.

It was shortly after Einstein formulated the equations of General Relativity (1915) when the idea could be restated in a much more satisfactory way. As we have seen, this theory teaches that matter bends space-time and causes the deflection of light, because light, without mass, must also comply with the equivalence principle. Therefore, in this theoretical framework, instead of stating that light is gravitationally attracted, we say that light rays follow the geodesic trajectories of curved space-time. When, after a gravitational collapse, this curvature of space-time converges towards a singularity, the geodesic trajectories inevitably lead towards it any material particle and also any photon that is less distant from the singularity than a certain limit value.

In 1967 Hawking (1942-2018) and Penrose (1931- ) ratified and expanded Oppenheimer’s prediction by demonstrating, based on Einstein’s equations, that in certain cases a star could not be prevented from ending up suffering a complete gravitational collapse. . The idea took even more strength with the advances that led to the discovery of pulsars at that time and in 1969 the term “black hole” was coined, after Wheeler (1911-2008) used it at a meeting of cosmologists held in New York. Penrose’s work rigorously demonstrated that the formation of black holes is a robust prediction of the General Theory of Relativity and is still considered the greatest contribution to the field of General Relativity since Einstein. For this reason, Penrose has been awarded half of the Nobel Prize in Physics in 2020.

On the other hand, it should be known that the solution found by Schwarzschild is applicable only to the simplest type of black hole that can be conceived, with spherical and static symmetry (Schwarzschild black hole), and whose configuration therefore depends on a only parameter: its mass M. Reality is somewhat more complex and to overcome this limitation, other types of black holes were modeled: Kerr’s black hole. in rotation (depends on two parameters: mass and angular momentum); the Reissner-Nordström black hole, electrically charged and static (it also depends on two parameters: mass and charge) and the Kerr-Newman black hole, charged and rotating (it depends on the three parameters).

We have already said that supermassive black holes are surrounded by an accretion disk that, when it suffers the enormous gravitational effects produced by the great central mass, drops part of its mass into the abyss of the hole. The region of interaction between the disk and the hole is a tremendously energetic zone in which large jets of matter can originate that are ejected from the polar zones and where very high-energy X-ray emissions are produced. The detailed analysis of this X-ray radiation thus makes it possible to investigate in detail the structure of the region closest to the black hole.