The strongest evidence for black holes comes from rapidly flickering X-ray sources. This observation indicates that a. X-rays are not light, because even light cannot escape from a black hole. b. the X-rays are coming from a very small source-so small that it could be a black hole. c. black holes are (at least sometimes) surrounded by hot gas. d. black holes have a very high temperature.

X-rays are a form of electromagnetic radiation, just like visible light, but they have much higher energy. In the cosmos, X-ray sources are often associated with extremely energetic and high-temperature environments.
Black holes are a prime source of X-rays in space. When material from a surrounding environment falls towards a black hole, it accelerates and heats up due to gravitational forces.
This high-energy state causes the emission of X-rays, which we can detect using special telescopes.
The rapid flickering of these X-ray sources often indicates the presence of a black hole because it suggests that the source of the X-rays is very small and very energetic, consistent with material being siphoned into a black hole.

Gravitational pull is the force that attracts an object towards the center of another object. The strength of this force depends on the masses of the objects and the distance between them.
Black holes exhibit an incredibly strong gravitational pull due to their immense mass packed into a very small volume.
This force is so powerful that it can drag in matter from its surrounding environment, creating an environment where the gravitational forces are extremely high.
The closer you get to a black hole, the stronger this gravitational pull becomes, eventually leading to a point where not even light can escape, known as the event horizon.
It's this dramatic effect of gravitational pull that can cause surrounding material to emit X-rays before it crosses the event horizon.

An accretion disk is a structure formed by material falling into a gravitational source, like a black hole or a young star.
The material orbits the black hole in a spiraling manner and gets heated due to the immense gravitational forces.
The heat generated in the accretion disk is so intense that it causes the emission of X-rays and other high-energy radiation.
The rapid rotation and immense gravitational forces can lead to flickering effects in the X-ray emissions.
The presence of these flickering X-rays is one of the tell-tale signs of an accretion disk around a black hole.
Studying accretion disks helps scientists understand the behavior of material under extreme gravitational forces and the fundamental properties of black holes.