Astronomers discover closest black hole to Earth.


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Astronomers using the International Gemini Observatory, operated by NSF's NOIRLab, have discovered the closest-known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its close proximity to Earth, a mere 1600 light-years away, offers an intriguing target of study to advance our understanding of the evolution of binary systems. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

The most extreme things in the cosmos are black holes. All huge galaxies presumably have supermassive versions of these unfathomably dense objects at their cores. There are an estimated 100 million stellar-mass black holes in the Milky Way alone, which are significantly more prevalent and weigh five to one hundred times as much as the sun.

But thus far, only a small number have been identified, and almost all of them are "active"—that is, they emit brilliant X-rays as they devour matter from a nearby star partner, in contrast to dormant black holes, which do not.

The nearest black hole to Earth has been named Gaia BH1 by astronomers using the Gemini North telescope on the island of Hawaii, one of the twin telescopes of the International Gemini Observatory, which is run by NSF's NOIRLab. It is three times closer to Earth than the previous record-holder, an X-ray pair in the constellation of Monoceros. This dormant black hole is around 10 times as massive as the sun and is situated about 1,600 light-years distant in the constellation Ophiuchus.

Exquisite studies of the motion of the black hole's partner, a sun-like star that circles the black hole at around the same distance as the Earth orbits the sun, allowed for the new finding.

According to Kareem El-Badry, an astronomer from the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Astronomy and the paper's primary author, "Take the solar system, put a black hole where the sun is, and the sun where the Earth is, and you have this system."

"Even though there have been several reports of such systems being detected, virtually all of these reports have now been debunked. The discovery of a sun-like star in a broad orbit around a black hole in our galaxy with a stellar mass is new information."

The few stellar-mass black holes that have been found were revealed by their energizing interactions with a partner star, despite the fact that there are probably millions of them wandering the Milky Way Galaxy. Superheated material from a nearby star spirals in toward the black hole, where it produces intense X-rays and material jets. When a black hole is inactive (i.e., not actively feeding), it simply merges into its surroundings.

For the last four years, El-Badry has been looking for latent black holes using a variety of datasets and techniques. This is the first time the search has yielded results, despite the fact that prior searches of mine and others have brought up a variety of binary systems that pass for black holes.

Data from the Gaia satellite of the European Space Agency were first examined by the researchers to determine the system's probable presence of a black hole. Gaia caught the minuscule deviations in the star's velocity brought on by an invisible huge object. El-Badry and his colleagues used the Gemini Multi-Object Spectrograph instrument on Gemini North to study the system in further detail. This device accurately determined the companion star's orbital period by measuring the companion star's velocity as it circled the black hole.

The team was able to determine that the binary system's center body is a black hole with a mass about equal to our sun thanks to the Gemini follow-up observations, which were important in limiting the orbital motion and, therefore, the masses of the two components.

El-Badry said, "Our Gemini follow-up measurements proved without a shadow of a doubt that the binary comprises a typical star and at least one inactive black hole. There is no conceivable astrophysical explanation for the system's observed orbit that does not include at least one black hole, according to our research.

As they only had a little window to conduct their follow-up observations, the team relied not only on Gemini North's excellent observational skills but also on Gemini's capacity to provide data on short notice.

"Only one week had passed since the two objects were furthest apart in their orbits when we first learned that the system had a black hole. For a binary system to provide correct mass estimations, measurements at this moment are crucial "El-Badry added. "The success of the initiative depended heavily on Gemini's capacity to provide quick observations. We would have had to wait another year if we had missed that little opportunity."

The unique configuration of the Gaia BH1 system is difficult to explain using astronomers' existing concepts of the development of binary systems. The progenitor star, which subsequently evolved into the newly discovered black hole, would have had a mass at least 20 times that of the sun.

It would have had a short lifespan of a few million years. If both stars had formed simultaneously, this enormous star would have swiftly evolved into a supergiant, inflating and swallowing the second star before it had a chance to develop into a proper, main-sequence star like our sun, which burns hydrogen.

The findings of the black hole pair suggest that the solar-mass star could not have survived that incident and ended up as an apparently normal star. The solar-mass star should have ended up on an orbit that is substantially closer to what is actually seen, according to all theoretical models that do allow for survival.

This may imply that there are significant knowledge gaps regarding the formation and evolution of black holes in binary systems, as well as the presence of an undiscovered population of dormant black holes in binary systems.

El-Badry continued by saying that it is "interesting that this system is not readily accommodated by typical binary evolution theories." The formation of this binary system and the number of these latent black holes in the universe are two of the many mysteries raised by this.

"Gemini North has provided strong evidence for the nearest black hole to date and the first pristine black hole system, uncluttered by the usual hot gas interacting with the black hole," said NSF Gemini Program Officer Martin Still. Gemini North is a part of a network of space- and ground-based observatories.

The results also pose a conundrum to be solved: why is the companion star in this binary system so typical while having a shared history with its strange neighbor? This might possibly portend future discoveries of the expected latent black hole population in our Galaxy. 

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