Astronomers discover a dormant black hole outside the Milky Way for the first time!

First explain what a dormant black hole is.

Black holes themselves do not emit light. Those detected in the Milky Way usually have a companion star, and they are in a binary system. A black hole in a binary system can form an accretion disk around the black hole by accreting material from its companion star. The accretion disk can generate radiation, and the energy of the radiation will be concentrated in the X-ray band. This type of accreted black hole binary is called a black hole X-ray binary. The famous black hole binary system, Cygnus X-1 (CygnusX-1) is a black hole massive X-ray binary star in the Milky Way that is accreting material from a companion star to form an accretion disk and emit X-ray radiation.

There are usually two ways for a black hole to accreulate the material of a companion star. One is that the companion star is filled with Roche lobes, and the material transfers the material through the inner Lagrangian point of the binary system (as shown in the figure above); the other way is the black hole itself. Gravity captures the stellar wind blown by the companion star (pictured below). The matter accreted by the black hole will not be directly sucked into the black hole, but will form an accretion disk structure around the black hole because of its angular momentum. It's like we know that there is gravitational force between the sun and the earth, but the earth is not sucked into the sun because of the existence of angular momentum. There will be a transfer or dissipation of angular momentum inside the accretion disk, so that the matter in the accretion disk will fall into the black hole. There is a viscous effect in the accretion disk, and the huge gravitational potential energy of the accreting matter is converted into the source of the radiation energy of the black hole's accretion disk

For a dormant black hole, we can think that it may not have undergone material transfer, or the material transfer has ended, and the companion star no longer provides a material source for the black hole. Without the accretion process of matter, there would be no radiation from the accretion disk, so we would not be able to find the traces of black holes in the radiation in the electromagnetic band

How this dormant black hole was discovered

Although this black hole is in a dormant state, it will have a gravitational interaction with its companion star, which is actually a process of revolving around each other.

During the orbit of the binary star, the orbital velocity in the direction of our line of sight will cause a red-shift or a blue-shift of the radiation spectrum of the companion star.

This effect is also very common in life. For example, when a sprinkler is playing music, you will find that the music pitch changes when it is close to you and away from you. A change in pitch is a change in frequency.

A dormant black hole has never been "definitely" detected outside the Milky Way before, the researchers said. More than a dozen discoveries about black hole binary systems have been questioned in the past two years, CNN reported, citing research, but astronomers involved in the study said they ruled out all other possibilities and determined that the "invisible" The "object" is a black hole.

The universe is vast and infinite, and there are many unknowns. Compared with the universe, human beings are an insignificant existence, but it is this tiny existence that has infinite curiosity about this armyless universe. And desire, has been diligently trying to understand the mysteries of the universe.