Thousands of stellar-mass black holes are present near the galactic center

Observation confirms presence of X-ray binaries composed of a black hole and another stellar remnant.
By Laurel Kornfeld | May 11, 2018
Using NASA'sChandra X-ray Observatory, a team of astronomers has found evidence for the presence of thousands of stellar-mass black holes in the center of the Milky Way.

Stellar-mass black holes are the remnants of massive stars that died in supernova explosions. The masses of the black holes range from five to 30 times that of the Sun.

Led by Chuck Hailey of Columbia Universityin New York, a research team used Chandra to search for binary systems near the galactic center. These systems are composed of one black hole and one companion that is either a normal star, a neutron star, which is also a supernova remnant, or a white dwarf, the remnant of a low-mass Sun-like star.

According to many studies of stellar dynamics, the galactic center is the home of as many as 20,000 stellar-mass black holes, which have slowly drifted inward over very long time scales. The Chandra data is the first confirmation of these black holes being abundant near the center of the Milky Way.

In binary systems, black holes in close orbits around normal or neutron stars or white dwarfs pull in material from their companions, which heats up to millions of degrees and emits X-rays before being sucked into the black holes. These pairs are known as X-ray binaries.

After finding sources of X-ray spectra similar to those emitted by X-ray binaries within 12 light years of the galactic center, known as Sagittarius A*, the researchers located 14 such systems within a radius of three light years of the center.

Two of these 14 systems likely contain one neutron star and many contain a white dwarf.

Because only the brightest X-ray sources are detectable, the researchers believe there are actually between 300 and 1,000 faint binary black hole systems and 10,000 to 40,000 single black holes near the galactic center that currently cannot be seen.

Many single stellar-mass black holes are also likely present near Sagittarius A* but are invisible and cannot be detected.

It is possible that some of the sources observed are not stellar-mass black holes at all but millisecond pulsars, or rapidly spinning neutron stars.

A better understanding of black hole binaries near the galactic center will provide scientists with insight into the formation process of X-ray binary systems as well as predict gravitational wave events produced by the binaries.

Findings of the study have been published in the journal Nature.






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