Astronomers have found that the merging of black holes can be accompanied by a flash in the electromagnetic range. This should happen in the case of the merger of stellar-mass objects in orbits around supermassive black holes, and recent estimates indicate a noticeable probability of such mergers.
Gravitational antennas detect periodic space-time oscillations that occur when large massive objects merge, such as black holes and neutron stars. However, the properties of black holes are primarily determined by their masses and spins; therefore, when such bodies collide in a vacuum, no signal of other types of radiation should arise. This complicates the study of these objects since at the moment the bulk of the information in astronomy is obtained through electromagnetic waves.
However, if the merging of black holes occurs in the environment of dense layers of ordinary matter, then theoretically there may be a noticeable signal. This situation can be realized, for example, in an accretion disk around a supermassive black hole. According to recent work, small black holes of stellar masses can accumulate in this area, which then merges, being surrounded by streams of hot plasma.
In this recent study, astrophysicists from the USA and Great Britain with the participation of Barry McKernan from the American Museum of Natural History evaluates the energy release of electromagnetic waves and the possibility of their registration in the event of such a merger. The reason for the appearance of radiation is the collision of gas flows after confluence, since the mass of the resulting black hole is noticeably less than the sum of the masses of the original, as well as due to the expected high speed of its movement.
The area of space in which gravity of an astronomical object dominates is called its Hill sphere. The radius of this zone for the system of two black holes before the merger is greater than for the product of their merger since part of the mass (usually about five per cent) is spent on the generation of gravitational radiation. A quick change in the Hill radius has several consequences.
Firstly, the part of the gas that was previously in stable orbits now moves too fast to hold a new black hole – this substance starts moving from the center of mass and collides with the more distant part of the accretion disk of the supermassive black hole. Secondly, part of the gas is now outside the Hill sphere and will also interact with larger-scale flows in the main disk. Thirdly, a new black hole quickly absorbs part of the gas with slow rotation after the propagation of the disturbance through the surrounding matter. A separate effect will occur in the case of the rapid movement of a new black hole – in this case, the gas will tend to follow the massive object, but will experience great resistance from the surrounding unperturbed disk material.
All these effects should lead to the appearance of a small bright spot against the background of radiation from the accretion disk of a supermassive black hole. The easiest way to record this process is if the original Hill radius was greater than the thickness of the disk – in this case, a hole will actually form in it, which will quickly fill with gas, which will lead to the appearance of a shock wave and a short burst of radiation. If the radius of Hill was less than the thickness of the disk, then the result strongly depends on its transparency. The authors conclude that it is most likely to capture the flash in the ultraviolet range.
The simultaneous observation of the merger of black holes in the form of gravitational waves and electromagnetic radiation will significantly increase the accuracy of determining the parameters of the system, such as coordinates and mass of objects. This will create more accurate models of the processes taking place and advance the development of gravitational antennas.