Astronomers have discovered a lonely black hole in the Milky Way

(ORDO NEWS) — An international team of astronomers has reported the first reliable detection of a single black hole flying through interstellar space.

A black hole was discovered thanks to the microlensing event, when a fairly massive object flies against the background of a star in the Milky Way and distorts the light from it. The discovery is described in detail in a preprint published on the arXiv website.

Single stellar mass black holes are much harder to detect than black holes in binary systems. The latter become visible due to companion stars that move around an invisible object or give it their substance, which generates an accretion disk and a strong X-ray flux.

In the case of single black holes, astronomers have to scan the sky for microlensing events, when the star’s brightness rises for a while. In this case, the black hole, which has a strong gravitational field, acts as a lens that collects the rays of light from the star.

In the new work, scientists have proposed a way to unambiguously establish that the gravitational lens is a black hole, and not some other object.

A massive compact object is expected to produce very long microlensing events, but the duration depends on the relationship between the Einstein radius of the lens, i.e. the light distortion angle, and the speed of the lens itself.

In other words, long-term microlensing can produce both a black hole with a very large Einstein radius and a very faint, slow-moving star of low mass.

This ambiguity can be eliminated by measuring both the photometry of the microlens (how much the brightness of the source has increased) and the astrometric shift, that is, by what angle (usually about a milliarcsecond) the light from the source has shifted.

Astronomers used the Hubble Space Telescope to make precise astrometry of a 270-day microlensing event seen in the direction of the galactic bulge.

The scientists also made ground-based observations of the change in the light curve, and also measured the annual parallax, that is, the change in the apparent position of the star and lens due to the rotation of the Earth around the Sun, since this affects the accuracy of photometry.

As a result, the researchers calculated the mass of the lens, which reaches 7.1 solar masses with an error of plus or minus 1.3 solar masses, and estimated the distance to it at 1.58 kiloparsecs (about five thousand light years).

The lens itself did not emit noticeable light, which, along with a large mass, confirms its nature as a black hole. A black hole moves through the interstellar medium at a low speed of 45 kilometers per second, which scientists attribute to a small push during a supernova explosion.

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