The First Triple Star System Found Containing a Black Hole

Neutron stars and black holes are the remnants of dead stars. They typically form as part of a supernova explosion, where the outer layers of an old star are violently cast off while the core of the star collapses to form the remnant. This violent origin can have significant consequences for both the remnant and the surrounding environment.

One thing that can happen is that the remnant can get a “natal kick,” which causes the remnant to speed away from the supernova remnant. We see this with some neutron stars, where we observe the neutron star leaving the remnant at speeds of more than 800 kilometers per second. We aren’t entirely sure what causes such large natal kicks, but they aren’t uncommon. One would assume the same thing could happen for stellar black holes. In fact, given the greater intensity of a black-hole forming supernova, you might think the kick would be even larger. But recent observations suggest that sometimes a stellar black hole can form with hardly any kick at all.

The observations focus on a black hole known as V404 Cygni. It has a mass about 10 times that of the Sun and is about 8,000 light-years away. It is also a microquasar. There is a small star that orbits V404 so closely that material is captured by the black hole. The captured material has created an accretion disk and jets similar to those formed by supermassive black holes in distant galaxies. It was discovered in 1938 and is easily observed in both visible and x-rays. With a decent telescope, you could even observe it from your backyard. Needless to say, V404 has been quite well studied.

But this new work found something new. The team identified a companion star orbiting the close binary. The star has been known for a while, but it wasn’t until we had detailed observations from the Gaia spacecraft that the team could prove it orbits the other two. It takes 70,000 years for the distant companion to make a single orbit, but it is gravitationally bound to the other two. So V404 Cygni is a triple system, not a binary one. Which is a bit strange. When the black hole formed, it should have been kicked away from the system. The close companion could have hung on, but the distant companion shouldn’t still be bound. So what gives?

When the team looked at the dynamics of the system, they found the natal kick of the black hole could have been no larger than 5 km/s. In astronomical terms, that’s essentially nothing. Therefore, V404 must have had no natal kick. If it formed from a supernova explosion, that would be unlikely. To figure out this mystery, the team looked at various models that might produce such a system. Everything from highly symmetric supernova explosions to direct collapse models where the black hole formed slowly and quietly rather than with a single big boom. It turns out the quiet approach is the most likely. It seems V404 gradually accumulated material from its close companion until it just collapsed to become a black hole, and it did so quietly enough for the third companion to go along for the ride.

Reference: Burdge, Kevin B., et al. “The black hole low mass X-ray binary V404 Cygni is part of a wide hierarchical triple, and formed without a kick.” arXiv preprint arXiv:2404.03719 (2024).

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