Two new research papers allow us to get closer to the space close to the event horizon and to form images of events in the region where the stable orbits closest to the black hole are located. The authors of both studies look at the periodic emissions that occur when black matter begins to absorb new matter.
Black holes themselves absorb all light outside their event horizon, and space outside such 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 torque and crashes into other matter in orbit around the black hole. Thus, although we cannot get an image of the black hole directly, we can draw some conclusions about its properties using the light from the environment that it creates.
This week, two research papers have been published that allow us to get closer to space close to the event horizon and to form images of events in the region where the stable orbits closest to the black hole are located. The authors of one of these papers 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.
Glow echo
The authors of both studies look at the periodic emissions that occur when black matter begins to absorb new matter. This substance is channeled into the hole through a flat structure centered in a black hole. This structure is called an accretion disk. As new matter appears, 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 of what these changes can tell us about the black hole and the space nearby.
In one of these papers, the attention of scientists is focused on a black hole with stellar mass, which is 10 times the mass of the Sun. In response to matter getting inside, one of these stars created a transient event called MAXI J1820 + 070. It got its name from the MAXI instrument on the ISS, which is designed to conduct astronomical observations in the X-ray range. Following the discovery of this event, it was possible to conduct new observations using the ISS equipment called NICER, which examines the internal composition of neutron stars. This equipment can make very fast measurements of X-rays emitted by astronomical sources, which allows you to effectively monitor short-term changes in an object.
In this case, the NICER instrument was used to analyze the "light echo". The point is that in addition to the accretion disk, black holes have a corona, which is a bubble of energetically charged matter located above and below the plane of the disk. This corona itself emits X-rays that can be detected with instruments. But these X-rays also hit 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.
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Solving the mystery
In this case, the light echo helped solve the puzzle. Images taken from superdense black holes in the center of galaxies indicate that the accretion disk has extended along the closest stable orbit to the black hole. However, measurements of stellar mass black holes indicate that the edges of the accretion disk are much further away. Since physical properties are unlikely to change with size, these measurements have puzzled scientists somewhat.
A new analysis shows that there are both variable and constant properties in MAXI J1820 + 070 X-rays. Constant properties indicate that the accretion disk creating the echo 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 is displaced. The details of the constant signal indicate that the accretion disk is much closer to the black hole. Thanks to this, the new measurements come in full agreement with what we know about superdense versions of black holes.
Death of a star
In the superdense territory is the ASASSN-14li object, discovered during the automatic exploration of supernovae. This object had properties that are commonly found in an event called tidal disruption. During such an event, the black hole, by the force of its gravity, tears apart a star that is too close to it. However, subsequent observations showed that this signal has a rather strange structure. Every 130 seconds, it gave a burst for a short time.
This signal was not very different from the background against which the destruction of the star took place, but it was detected by three different instruments, which indicates that something is happening 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 rotation speed 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 reproduce simulations many times, testing various configurations of the 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 something is to the black hole, the slower the black hole rotates so that the object moves in orbit at the same speed. Thus, by calculating the closest possible orbit, scientists were able to determine the minimum value of the rotation speed.
The calculations performed indicate that the black hole rotates at least with such a speed that a point on its surface moves at a speed half the speed of light. (To give you a better idea, it should be said that superdense black holes can be so large that their radius is the same as the radius of the orbit of Saturn or Neptune.) If matter orbits a little further from the center, then so can the black hole accelerates its rotation.
We cannot yet obtain images of black holes directly, but studies have shown that numerous events occur in them, which can give us a lot of data 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 waiting in the wings to get into them. We're also starting to get information from gravitational wave observations that gives us an idea of the mass and rotation of colliding black holes. Taken together, these data remove a halo of obscurity from black holes, and they are no longer unexplored territory for us.
John Timmer
