Using data from Gaia, astronomers have just discovered a low-mass black hole orbiting a companion star. Credit: WANG song
Astronomers have discovered a light black hole that poses a bit of a cosmic enigma. Hypothetically, black hole masses can range from much less than a paper clip to at least tens of billions of times that of the Sun. But observations have revealed a strange paucity of black holes between about two and five times the mass of the Sun. At the moment, it is unclear whether these mini black holes are simply difficult to detect or even as rare as they seem.
The newly discovered black hole could offer clues. It falls right in the middle of the gap, weighing about 3.6 solar masses. A team of scientists found it thanks to a giant companion, a red giant star located about 5,800 light years from Earth. Although the star is only about 2.7 times more massive than the Sun, it is about 13 times larger and 100 times brighter.
Discovery of a black hole
Using data from the European Space Agency’s Gaia spacecraft, which tracks the positions of more than a billion stars in our galaxy over time, astronomers spotted the red giant circling an unseen partner. Because Gaia mostly tracks its own motion – 2D motion, or left, right, up and down – scientists have tracked ground-based telescopes to measure the star’s motion towards and away from us.
That information revealed something unexpected: two objects, together called G3425, in a very wide circular orbit around each other.
This is why it is perplexing. When the black hole formed, it would have rapidly lost mass during a supernova explosion. That weight loss should disrupt the gravity of the binary system.
“There is a basic idea in orbital mechanics that if a binary system loses half its mass it breaks free,” writes Phil Plait, an astronomer not involved in the study, in a newsletter. “In other words, the two objects separate.” While in this case the black hole’s weight loss would not have been extreme enough to unmoor it entirely, it would have produced a highly elongated orbit, not the nearly circular one seen by astronomers.
This also assumes that the supernova occurred uniformly. However, observations have shown that these explosions can be a bit unbalanced, sending the remaining object hurtling through space. This would certainly explain why so few low-mass black holes are found: they could be detached from their companion stars and left to wander the cosmic abyss alone and invisible. And it makes the unlikely cosmic duo G3425 that much more interesting.
Song Wang, an astronomer at the National Astronomical Observatories of the Chinese Academy of Sciences who co-led the study, suggests another possible explanation. G3425 may have started out as a triple system with two massive stars at the center and the red giant orbiting the pair. “The current black hole [may have] formed as a result of a merger of the inner track,” he says. “It is also possible that the central invisible object still contains two less massive compact objects.”
More and more curious
The researchers were hunting for binary systems in which one member was a light black hole to test the idea that such systems might not even be possible. But now that they have demonstrated a way to identify low-mass black hole systems, future observations could offer clues about how common these objects are and how they might form.
Scientists extracted G3425 from Gaia observations of nearly 1.5 billion sources, but there are more than 100 billion stars in our galaxy. According to Wang, this suggests that hundreds of other similar systems could be out there waiting to be discovered. Identifying and studying more of them, along with improving theories and simulations, could explain how these pairs evolved.
In his newsletter, Plait points out that this is not the first time scientists have identified binary systems that shouldn’t exist. A separate Gaia study targeted binary systems containing stars like our Sun, along with dense stellar corpses called neutron stars, which can also form following the collapse of a massive star.
Neutron stars are so faint that they are extremely difficult to spot when they are alone. But by watching the Sun-like stars wobble, the researchers were able to identify nearly two dozen dead stars invisibly tugging at their bright companions.
Again, it is unclear how these binaries exist.
“In principle, the progenitor of the neutron star should have become huge and interacted with the solar-type star during its late stage of evolution,” said study leader Kareem El-Badry, assistant professor of astronomy at Caltech and an adjunct scientist at the Max Planck Institute for Astronomy in Germany, in a press release. When that progenitor went supernova, it should have caused the two stars to explode in different directions.
“Clearly, there’s a piece we’re missing here, and my gut feeling is that it’s the same piece in both stories,” Plait says. “Some factor in the way stars evolve in a binary system is doing something unexpected, and it is doing similar things for black holes and neutron stars in large orbits around Sun-like stars.”
