What happens when a massive star dies? Conventional wisdom (and observational evidence) say that it can collapse to form a “stellar-mass” black hole. Astronomers detect black holes by the X-ray emissions they emit.
But, what if the black hole isn’t giving off high levels of X-ray emissions? Then, it could be a very rare object indeed: a dormant black hole. Not many of these have been seen. So, it’s exciting to know that a team of astronomers has found one. It’s called VFTS 243. They detected it in Very Large Telescope observations of stars in the Tarantula Nebula, in the neighboring Large Magellanic Cloud.
Finding a Needle in a Galactic Haystack
VFTS 243 is the first dormant black hole to be unambiguously detected outside the Milky Way Galaxy. While there are likely billions of stellar-mass black holes out there, dormant ones are much harder to find. “We identified a ‘needle in a haystack’,” said team leader Tomer Shenar, a Marie-Curie Fellow at Amsterdam University, the Netherlands.
It’s not for lack of looking, and astronomers have searched. “It is incredible that we hardly know of any dormant black holes, given how common astronomers believe them to be”, said team member Pablo Marchant of KU Leuven.
This newly found dormant black hole is at least nine times the mass of our Sun. It’s in a 10.4-day orbital dance with a hot, blue star 25 times the mass of the Sun. The black hole itself has about 9 solar masses. It’s considered dormant because it’s not emitting large amounts of X-rays. Normally, that would happen as material spirals into its accretion disk from the nearby companion star.
This artist’s impression shows what the binary system VFTS 243 might look like if we were observing it up close. The sizes of the two binary components are not to scale: in reality, the blue star is about 200 000 times larger than the black hole. Note that the ‘lensing’ effect around the so-called “dormant” black hole is shown for illustration purposes only, to make this dark object more noticeable in the image. The inclination of the system means that, when looking at it from Earth, we cannot observe the black hole eclipsing the star. Credit: ESO/L. Calçada
Implications for Black Hole Formation
Although astronomers know the general story about how a massive star evolves to become a stellar-mass black hole, there are many details of the process to explore. This discovery gives the team a new way to look at the black hole “factory”. Typically, a stellar-mass black hole forms as the core of a dying massive star collapses. However, for a dormant black hole, it’s not clear if the collapse was followed by a powerful supernova explosion. This new one may have taken a very different route to “black hole-hood”. In fact, there are some interesting clues that point to an alternate evolutionary path. First, it’s in a near-circular orbit and its motions indicate there was little ejected material from an explosion. In addition, there’s no evidence of a form of rebound called a “black hole kick” as the object collapsed. So, what happened?
“The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion,” said Shenar, who noted that the team’s discovery has implications for understanding the properties of core-collapse supernovae as well as gravitational-wave detections. “Evidence for this ‘direct-collapse’ scenario has been emerging recently, but our study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos
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