A startling finding regarding the black hole V404 Cygni is reshaping our comprehension of black holes, their possible companions, and their formation processes.
To date, many detected black holes seem to exist as pairs. These binary systems consist of a black hole and another entity—such as a star, a denser neutron star, or another black hole—that revolve around one another, influenced by the gravitational pull of the black hole, creating a tight orbital relationship.
However, a recent and unexpected finding is broadening our insights into black holes, the potential companions they may host, and their formation mechanisms.
In a study published in Nature, researchers from MIT and Caltech announce their first observation of a “black hole triple.” This new configuration features a central black hole actively consuming a small star that spirals in very closely to it every 6.5 days—akin to most binary systems. Surprisingly, there is also a second star that appears to be orbiting the black hole from a much greater distance. The experts estimate that this distant companion completes an orbit around the black hole every 70,000 years.
The fact that the black hole seems to exert gravitational force on something so far away raises intriguing questions regarding its own origins. Traditionally, black holes are believed to originate from the explosive demise of a star—a phenomenon called a supernova, wherein a star emits a vast amount of energy and light in a final explosion before collapsing into an invisible black hole.
Contrarily, the team’s discovery indicates that if the black hole in question came from a typical supernova, the energy discharged during its collapse should have expelled any loosely bound objects in its vicinity. Therefore, the second, distant star should not still be present.
Instead, the researchers propose that the black hole formed through a gentler mechanism known as “direct collapse,” where a star simply collapses in on itself without an explosive finale, avoiding significant disruption to surrounding loosely-bound, distant objects.
This new triple system’s inclusion of a far-off star points to a gentler birth for the black hole through this direct collapse method. While astronomers have observed more explosive supernovae for many years, the team argues that this new triple system may provide the first evidence of a black hole created via this quieter process.
“We typically think that black holes emerge from violent stellar explosions, but this discovery challenges that assumption,” expresses study author Kevin Burdge, a Pappalardo Fellow in MIT’s Department of Physics. “This system is incredibly exciting for understanding black hole evolution and raises questions about whether more triple systems are out there.”
The co-authors of the study from MIT include Erin Kara, Claude Canizares, Deepto Chakrabarty, Anna Frebel, Sarah Millholland, Saul Rappaport, Rob Simcoe, and Andrew Vanderburg, along with Kareem El-Badry from Caltech.
Combined Motion
The detection of the black hole triple was somewhat serendipitous. The physicists stumbled upon it while examining Aladin Lite, a database of astronomical observations collected from telescopes worldwide and in space. This online tool enables astronomers to search for images of specific areas of the sky captured by various telescopes operating at different energy and light wavelengths.
The researchers were primarily exploring the Milky Way galaxy for signs of new black holes. Out of curiosity, Burdge checked an image of V404 Cygni—a black hole located approximately 8,000 light-years from Earth, recognized as one of the earliest confirmed black holes back in 1992. Since then, V404 Cygni has been extensively studied, appearing in over 1,300 scientific articles, but none documented the observation made by Burdge and his team.
When Burdge examined optical images of V404 Cygni, he noticed two blobs of light that were surprisingly close. The first blob was identified as the black hole and an inner star that orbits closely around it. This proximity leads the star to shed some material onto the black hole, producing visible light. The second blob, however, was a previously overlooked source, which Burdge inferred to be emanating from a very distant star.
“Seeing two distinct stars at this distance indicates they must be tremendously far apart,” says Burdge, who calculated the outer star’s distance to be 3,500 astronomical units (AU) from the black hole (1 AU being the distance from Earth to the Sun). In simpler terms, the outer star is 3,500 times farther from the black hole than Earth is from the Sun, equivalent to 100 times the distance from Pluto to the Sun.
This leads to the inquiry of whether the outer star is in any way linked to the black hole and its inner companion. To find out, the researchers consulted Gaia, a satellite that has accurately tracked the movements of stars in the galaxy since 2014. By analyzing the motion of both inner and outer stars over the last ten years of Gaia data, the team noted that they moved in perfect unison compared to their neighboring stars. They deduced that the likelihood of such synchronized motion stands at about one in ten million.
“This is almost definitely not mere chance,” asserts Burdge. “We are observing two stars moving together because they are entwined by a weak gravitational connection. Thus, we confirm this is a triple system.”
Interconnected Forces
How, then, did this system come to be? If the black hole stemmed from a typical supernova, its explosive blast would have ejected the outer star long before now.
“Consider pulling a kite; instead of using a strong string, you are pulling with a fragile spider web,” explains Burdge. “If you pull too hard, the web breaks, and you lose the kite. Gravity acts like this barely-bound string that is quite weak; any significant disturbance to the inner binary could cause the loss of the outer star.”
Burdge conducted simulations to explore how this triple system could have developed and retained the outer star. At the beginning of each simulation, he included three stars (the black hole being the third prior to its transformation). He ran tens of thousands of simulations, each presenting a slightly altered scenario of how the third star could have evolved into a black hole, affecting the motion of the remaining two stars. Situations included a supernova, varying the energy’s amount and direction released, along with scenarios of direct collapse where the third star simply imploded to become a black hole, without releasing any energy.
“The majority of simulations indicate that the simplest way to create this triple system is through direct collapse,” notes Burdge.
Besides providing insights into the black hole’s origins, the outer star also unveils the age of the system. The researchers noted that this outer star is currently transitioning into a red giant, a stage that happens toward the end of a star’s lifecycle. Utilizing this stellar progression, they estimated the outer star’s age to be about 4 billion years. Given that stars born close to one another typically share a similar age, the researchers conclude that the black hole triple is also 4 billion years old.
“We have not been able to achieve this with an ancient black hole until now,” Burdge remarks. “Recognizing that V404 Cygni is part of a triple system opens up the possibility of its formation via direct collapse approximately 4 billion years ago, thanks to this discovery.”
This research was partly supported by the National Science Foundation.
