Closest Black Hole Found, Just 1,000 Light-Years From Earth
Even light cannot escape black holes, which are invisible to the naked eye and have no locally detectable features. Despite this, their impact on the environment makes them an ideal laboratory for testing physics in extreme conditions. They provide astronomers with an opportunity to test Einstein’s Theory of General Relativity, which states that the presence of gravity alters the curvature of space-time.
The closest black hole has been discovered, thanks to a team of astronomers led by the European Southern Observatory (ESO). The team discovered this black hole in a triple system in the Telescopium constellation, just 1000 light-years from Earth, using the ESO’s La Silla Observatory in Chile. This system, known as HR 6819, can be seen with the naked eye and could be one of many “quiet” black holes out there.
ESO scientist Thomas Rivinius led the study that describes their findings, which was recently published in the journal Astronomy & Astrophysics. Other team members included ESO scientists from Spain and Germany, the Astronomical Institute of the Czech Academy of Sciences, and Georgia State University’s CHARA Array.
The discovery was made as part of a study of binary pairs that used the La Silla Observatory’s MPG/ESP 2.2-meter telescope to observe nearby star systems. They discovered evidence of a third unseen object roughly 4.2 times the mass of the Sun while tracking the companion stars in this system, putting it in the “stellar” black hole range.
“We were totally surprised when we realized that this is the first stellar system with a black hole that can be seen with the unaided eye,”said Petr Hadrava, an Emeritus Scientist at the Czech Academy of Sciences in Prague and co-author of the study. In fact, the system is close enough that it can be seen without binoculars or a telescope in the southern hemisphere on a clear night.
Dietrich Baade, a co-author of the study and an Emeritus Astronomer at ESO in Garching, says:
“The observations needed to determine the period of 40 days had to be spread over several months. This was only possible thanks to ESO’s pioneering service-observing scheme under which observations are made by ESO staff on behalf of the scientists needing them.“
Unlike the few dozen stellar-mass black holes discovered so far in our galaxy, HR 6819 is one of the first to be discovered that does not interact violently with its surroundings – and thus is truly invisible. Despite this, the team was able to detect it and calculate its mass by observing the effect it has on the binary’s inner stellar companion’s orbit.
The black holes in nearly all previous cases interacted strongly with their surroundings, as evidenced by the powerful release of X-ray bursts. This interaction, for example, occurred as part of a black hole binary, in which a black hole drew material from a companion star. This material would fall into a disk around the black hole’s event horizon, then slowly accrete onto its surface, emitting radiation.
Only a few of these black holes have been discovered in the Milky Way so far. Given the Milky Way’s age, scientists believe that many more stars must have collided into black holes during its existence. As a result, the discovery of a “quiet” black hole in HR 6819 could provide insight into the location of these yet-to-be-observed black holes.
“There must be hundreds of millions of black holes in the universe, but we only know about a few. Knowing what to look for should help us find them faster,” Rivinius said, emphasizing that this discovery was likely just “the tip of an exciting iceberg.”
The team also noticed a connection between this system and what spectroscopic time-series observations revealed about another system. This binary star system, known as LB01, is about 7,000 light-years away from Earth in the constellation Gemini. Until now, astronomers thought it was composed of a star and an unseen BH companion with a mass of about 70 Solar masses.
The team demonstrated how the mass of the BH could be better constrained and more consistent with known stellar black holes by assuming the presence of a third object (another unseen BH). As co-author of the paper Marianne Heida, a postdoctoral fellow at ESO, said:
“We realised that another system, called LB-1, may also be such a triple, though we’d need more observations to say for sure. LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist. By finding and studying them we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole.“
Furthermore, research into triple systems may reveal more about the types of violent mergers that result in strong gravitational waves (GW). In a system where a distant outer object orbits an inner pair, the outer object could have a gravitational impact on the other two, causing a merger and the release of GWs.
This is particularly true if the inner pair is made up of two black holes or a black hole and a neutron star. “In sadness and grateful appreciation of his never-failing alertness that also triggered this work,” the authors dedicated their paper to the memory of Stan Tefl, an ESO and ALMA astronomer who died in 2014.