This discovery will help to learn more about the nature and formation of systems with double stars.
Astronomers have made an amazing discovery about what lies deep in the depths of our Galaxy. For example, researchers at the Chandra X-ray Observatory have found evidence of thousands of black holes in the bowels of the Milky Way.
The objects of the discovery are black holes of stellar mass, which usually weigh between five and 30 times the mass of the Sun. They were discovered just three light years away - a relatively short distance on cosmic scales - from a supermassive black hole in the center of the galaxy known as Sagittarius A * (Sgr A *).
Theoretical studies of stellar dynamics inside the Galaxy have shown that a large number of black holes of stellar mass - up to 20 thousand objects - can gradually drift towards Sagittarius A * and gather around it. A recent analysis using data from Chandra was the first evidence of such a cluster of black holes.
The black hole itself is invisible. However, a black hole - a neutron star, in fact - in close orbit with a star, will attract gas from the latter (astronomers call such systems "X-ray binaries"). In turn, the material falls onto the disk and heats up to millions of degrees, producing X-rays, before finally disappearing into the black hole. Some X-ray binaries are similar to the point sources in Chandra's images.
A black hole pulls in gas from a nearby star
A team of researchers led by Chuck Haley of Columbia University in New York used the Chandra data to search for X-ray binaries with black holes in the vicinity of Sagittarius A *. They studied the X-ray spectrum coming from sources within 12 light years from the center of the Galaxy.
The team then selected sources of the spectrum similar to those emitted by known X-ray binaries, in which there is quite a lot of low-energy X-rays. Thanks to this method, 14 X-ray binaries were discovered within three light years of Sagittarius A *. Two sources of radiation with putative neutron stars were subsequently excluded from the analysis due to the detection of characteristic flares in past studies.
The dozen remaining X-ray binaries are indicated by red circles in the tagged version of the image. Other sources of relatively high levels of radiation are marked in white - mainly systems with white dwarfs.
Haley and his collaborators concluded that most of these dozen systems contain black holes. The variability they have shown over the years is different from what was originally expected from X-ray binaries that include neutron stars.
Most likely, only the brightest of these black hole systems can be detected at Sagittarius A *. In other words, the results of this study suggest that many fainter, undiscovered X-ray binaries with stellar mass black holes have accumulated around the center of the Milky Way.
This population of black holes with companion stars near Sagittarius A * may provide new data on the formation of X-ray binaries. The discovery is likely to also help in future studies of gravitational waves. Knowing the number of black holes in the center of a typical galaxy, one can understand how many gravitational-wave events are associated with them.
In the future, this discovery will help to learn more about gravitational waves.
Sagittarius A * may have a much larger population of stellar mass black holes without companion stars. According to theoretical studies by Alexei Generozov from Columbia and his colleagues, there are from 10 to 40 thousand black holes in the center of the Galaxy.
And although the authors of the study adhere to the option of the presence of black holes in the found systems, they do not exclude that half of them may contain millisecond pulsars - very rapidly rotating neutron stars with a strong magnetic field.