Reviewed by Lexie CornerOct 24 2024
In Nature, astrophysicists from Caltech and MIT report the first observation of a "black hole triple," a system consisting of three stars, one of which is a black hole. Initially, it was believed that this black hole had only one companion star, but the discovery revealed a second partner.
Depicted in this artist's rendering is the central black hole, V404 Cygni (black dot), in the process of consuming a nearby star (orange body at left), while a second star (upper white flash) orbits at a much farther distance. Image Credit: Jorge Lugo
In the system known as V404 Cygni, the black hole is consuming a nearby small star that is spiraling in quickly and closely, while a newly discovered third star orbits the black hole from a much greater distance. This distant companion takes approximately 70,000 years to complete one orbit around the black hole.
The tertiary star has actually been hiding in plain sight for more than 30 years. Dozens of other papers have noticed that there is another star 'next to' V404 Cygni on the sky, but they just all assumed the star was a chance alignment. Only with precise astrometry from [the European Space Agency's] Gaia mission did it become clear that the star is actually at the same distance and is moving in the same direction as V404 Cygni, meaning that it's gravitationally bound to it.
Kareem El-Badry, Assistant Professor, California Institute of Technology
El-Badry is the second author of a recent study published in Nature that details these findings. Kevin Burdge, a Caltech alumnus and MIT Department of Physics Pappalardo Fellow (Ph.D. '21), is the first author.
The discovery raises a critical question: how can a black hole exert a gravitational pull on an object so distant? A black hole is typically thought to form from the violent explosion of a dying star, known as a supernova. Such an explosion would likely eject any loosely bound objects. As a result, the survival of the third star in the V404 Cygni system seems improbable.
The team, however, suggests that the black hole may have formed through implosion, where a star collapses in on itself to create a black hole without a final explosive supernova. This more gradual process could explain how the black hole retains a distant companion.
It is unclear if any more black hole triples can be found.
El-Badry added, “Either we got very lucky, or tertiaries are common. If they are common, that might solve some of the long-standing questions about how black hole binaries form. Triples open up evolutionary pathways that are not possible for pure binaries. People have actually predicted before that black hole binaries might form mostly through triple evolution, but there was never any direct evidence until now.”
Journal Reference:
Burdge, K. B. et. al. (2024) The black hole low-mass X-ray binary V404 Cygni is part of a wide triple. Nature. doi.org/10.1038/s41586-024-08120-6