Astronomers cannot actually see the black hole candidates in close binary systems. How, then, do they know that these candidates are not white dwarfs or neutron stars?

Astronomy is a branch of science that explores celestial objects and the universe as a whole. Within this field, black holes represent some of the most intriguing and mysterious phenomena. A black hole is a region of spacetime where gravity is so strong that nothing, no particles or even electromagnetic radiation such as light, can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. These cosmic enigmas are usually detected by their interaction with other matter and the gravitational effects they have on their surroundings.

Because black holes do not emit light, they cannot be seen through traditional optical telescopes. Astronomers instead look for their presence in binary systems, where one star is visible and the other is not. If a star appears to be orbiting around an invisible partner, especially at high speeds, it suggests that the partner might be a black hole. To confirm such suspicions, astronomers rely on additional observations and techniques, such as X-ray detection, which provides further evidence of these invisible objects in the cosmos.

The gravitational effects of black holes are so powerful they dominate their immediate environment. The gravity of a black hole is strong because its mass is compressed into a very small space at its center, called the singularity. Anything that gets too close to the event horizon, the boundary around the black hole, will be inexorably drawn in and consumed.

In binary systems, these gravitational effects can be observed indirectly through the motion of the companion star. The star's orbit will often be highly elliptical, and the velocity at which it moves can be used to determine the mass of the unseen companion. If the mass is calculated to be above the theoretical maximum mass for a neutron star, roughly 2 to 3 solar masses, it is likely to be a black hole. This mass estimation is a crucial step in distinguishing black holes from other dense objects like white dwarfs and neutron stars, which, while compact, do not exhibit the same degree of gravitational influence.

X-ray detection is an essential tool in the identification of black holes within binary systems. When a star in a binary system orbits close to a black hole, the intense gravitational pull of the black hole can strip away material from the star. This material spirals towards the black hole, forming an accretion disk as it heats up to incredibly high temperatures, releasing energy in the form of X-rays. Space telescopes equipped with X-ray detectors can observe these high-energy emissions, even from vast distances.

Unlike visible light, X-rays can penetrate through the dense, hot regions surrounding a black hole, providing a clear indication of the violent processes taking place. By measuring both the intensity and the pattern of these X-ray emissions, astronomers can infer the presence of a black hole and gather details about its properties such as mass, spin, and the rate at which it is consuming matter from its companion star. X-ray astronomy has revolutionized our understanding of black holes and has been instrumental in confirming the existence of these remarkable objects in the universe.