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Jan 17 2019

Ars Technica (USA): getting images closer to the event horizon

Two new research projects allow us to get closer to the space located near the event horizon, and to form images of events in the area where the stable orbits are closest to the black hole. The authors of both studies consider periodic emissions that occur when black matter begins to absorb a new substance.

New studies of matter, ready to fall into a black hole. 

01/17/2019 

John Timmer (JOHN TIMMER) 

The black holes themselves absorb all the light outside their event horizon, and the space outside such an event horizon usually emits such light in large quantities. This is due to the fact that matter falling into a black hole has a huge energy charge. It loses the rotational moment and crashes into another matter in orbit around the black hole. Thus, although we cannot directly capture an image of a black hole, we have the opportunity to draw some conclusions about its properties by taking advantage of the light from the environment it creates. 

This week, two research papers were published that allow us to approach the space near the event horizon and form images of events in the area where the stable orbits are closest to the black hole. The authors of one of these works came to the following conclusion: a supermassive black hole rotates so fast that a point on its surface moves at a speed equal to about half the speed of light. 

Echo glow 

The authors of both studies consider periodic emissions, which occur when black matter begins to absorb a new substance. This substance is directed into the hole through a flat structure centered in a black hole. This structure is called the accretion disk. With the advent of new matter, the disk heats up, making the black hole brighter. Because of this, changes occur in the surrounding space. The authors of both studies are looking for an answer to the question that these changes can tell us about the black hole and the space nearby. 

In one of these works, the attention of scientists is focused on a black hole of stellar mass, which is 10 times greater than the mass of the Sun. In response to matter getting inside one of these stars created a transient event called MAXI J1820 + 070. It received its name from the MAXI instrument located on the ISS, which is intended for conducting astronomical observations in the X-ray range. Following the discovery of this event, new observations were made with the aid of the ISS equipment called NICER, which explores the internal composition of neutron stars. This equipment can perform very fast measurements of x-rays emitted by astronomical sources, which allows you to effectively monitor short-term changes in the object. 

In this case, the NICER instrument was used to analyze the ‘light echo’. The fact is that in addition to the accretion disk, black holes have a corona, which is a bubble of energetically charged substance located above and below the plane of the disk. This crown itself radiates X-rays, which can be detected using instruments. But these X-rays also fall into the accretion disk, and some of them are reflected in our direction. Such a light echo can tell us some details about the accretion disk. 

Solving the mystery

In this case, the light echo helped to solve the riddle. Images obtained from superdense black holes in the center of galaxies indicate that the accretion disk stretched along a stable orbit closest to the black hole. However, measurements of black holes with stellar mass indicate that the edges of the accretion disk are much further. Since physical properties are unlikely to change with resizing, these measurements somewhat puzzled scientists. 

A new analysis shows that the MAXI J1820 + 070 X-ray radiation has both variable and permanent properties. Permanent properties suggest that an echo-creating accretion disk does not change its location at all. And the variable properties indicate that when a black hole devours matter, its corona becomes more compact, and therefore the x-ray source shifts. Details of the permanent signal indicate that the accretion disk is much closer to the black hole. Due to this, new measurements are in full compliance with what we know about super-dense versions of black holes. 

Death of a Star 

The ASASSN-14li object, found during an automatic investigation of supernovae, is located in the superdense territory. This object had such properties that are usually inherent in the event called “tidal destruction”. During such an event, a black hole, by the force of its attraction, tears apart a star that is too close to it. However, subsequent observations have shown that this signal has a rather strange structure. Every 130 seconds, he briefly gave a surge. 

This signal was not very different from the background on which the star was destroyed, but it was discovered by three different devices, and this indicates that something happens periodically. The simplest explanation is that part of the star fell into orbit around the black hole. The frequency of such orbits depends on the mass and speed of rotation of the black hole, as well as on the distance between the black hole and the object orbiting around it. In other ways, the rotation of a black hole is difficult to measure, and therefore scientists repeatedly reproduce simulation modeling, testing various configurations of a black hole system. 

The mass of a black hole is determined based on the size of the galaxy in which it is located. There is a simple relationship between the speed of rotation and the orbital distance: the closer such a thing is to the black hole, the slower the black hole rotates so that the object moves in orbit at the same speed. Thus, having calculated the closest possible orbit, the scientists were able to determine the minimum value of the rotation speed. 

Calculations indicate that a black hole rotates at least at a speed at which a point on its surface moves at a speed two times smaller than the speed of light. (To give you a better idea, you should say that superdense black holes can be so large that they have the same radius as the orbit of Saturn or Neptune.) If matter orbits a little further from the center, then the black hole too accelerates its rotation. 

We can’t get images of black holes directly yet, but our studies show that numerous events take place in them that can give us a lot of information about their behavior in the Universe. And this allows us to make certain conclusions about the properties of the black holes themselves, as well as about the matter, which is waiting for its time to get into them. We are also starting to receive information from observations of gravitational waves, which gives us an idea of ​​the mass and rotation of colliding black holes. In the aggregate, these data remove the halo of obscurity from black holes, and they are no longer an unknown territory for us.

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