The first discovery of what appears to be a deceptive black hole moving through the Milky Way, unveiled earlier this year, has just received important validation.
A second team of scientists, conducting a separate, independent analysis, came to almost the same discovery, adding weight to the idea that we had potentially identified a deceptive black hole wandering the galaxy.
Led by astronomers Casey Lam and Jessica Lou of the University of California, Berkeley, the new work came to a slightly different conclusion. Given the mass of the object, according to the new study, this could be a neutron star, not a black hole.
However, this means that we may have a new tool for searching for “dark”, compact objects that cannot otherwise be found in our galaxy, by measuring the way their gravitational fields distort and distort the light of distant stars as they pass through in front of them, called gravitational microlens.
“This is the first free-floating black hole or neutron star detected by gravitational microlensing,” says Lou.
“With microlensing, we can explore these lonely, compact objects and weigh them. I think we’ve opened a new window to these dark objects that can’t be seen any other way.”
Black holes are thought to be collapsed nuclei of massive stars that have reached the end of their lives and discarded their outer material. Such precursors of black holes – larger than 30 times the mass of the Sun – are thought to live relatively short lives.
Therefore, according to our best estimates, there should be between 10 million and 1 billion black holes with stellar mass, which float peacefully and quietly around the galaxy.
But black holes are called black holes for a reason. They do not emit light that we can detect unless material falls on them, a process that generates X-rays from the space around the black hole. So if a black hole just hangs without doing anything, we have almost no way to find it.
almost. What the black hole has is an extreme gravitational field, so powerful that it distorts any light that passes through it. For us as observers, this means that we can see a distant star look brighter and in a different position than it looks normal.
This is exactly what happened on June 2, 2011. Two separate microlens studies – the Optical Gravitational Lens Experiment (OGLE) and the Astrophysics Microlens Observations (MOA) – independently recorded an event that eventually peaked on July 20th.
This event was called MOA-2011-BLG-191 / OGLE-2011-BLG-0462 (abbreviated to OB110462) and because it was unusually long and unusually bright, scientists set out to take a closer look.
“How long the illumination lasts is a hint at how massive the lens in the foreground is, which bends the light of the background star,” Lam explains.
“Long events are more likely due to black holes. However, this is not a guarantee, as the duration of the lighting episode depends not only on how massive the foreground lens is, but also on how fast the foreground lens and the background star move. each other.
“However, by obtaining measurements of the visible position of the background star, we can confirm whether the lens in the foreground is really a black hole.”
Illustration showing Hubble viewing a microlens event. (NASA, ESA, STScI, Joseph Olmsted)
In this case, the observations of the region were made eight separate cases with the help of the Hubble Space Telescope until 2017.
From an in-depth analysis of these data, a team of astronomers led by Kailash Sahu of the Space Telescope Science Institute concluded that the culprit was a microlensing black hole 7.1 times the mass of the Sun at a distance of 5,153 light years. far away.
Lou and Lam’s analysis now adds more data from Hubble, taken recently in 2021. Their team found that the object is slightly smaller, between 1.6 and 4.4 times the mass of the Sun.
This means that the object can be a neutron star. This is also the collapsed core of a massive star that starts between 8 and 30 times the mass of the Sun.
The resulting object is supported by something called neutron degeneration pressure, in which neutrons do not want to occupy the same space; this prevents it from collapsing completely into a black hole. Such an object has a mass limit of about 2.4 times the mass of the Sun.
Interestingly, no black holes were found below about 5 times the mass of the Sun. This is called a lower difference in mass. If the work of Lam and her colleagues is correct, it means that we could find an object with less mass on our hands, which is very annoying.
The two teams returned with different masses for the lens object, as their analyzes yielded different results for the relative motions of the compact object and the lens star.
Sahu and his team found that the compact object was moving at a relatively high speed of 45 kilometers per second as a result of a natal impact: a unilateral supernova explosion could send the collapsing nucleus to accelerate.
However, Lam and her colleagues had 30 kilometers per second. This result, they say, suggests that a supernova explosion may not be necessary for the birth of a black hole.
It is currently impossible to draw a definite conclusion from OB110462 which estimate is correct, but astronomers expect to learn a lot from the discovery of more of these objects in the future.
“Whatever it is, the object is the first dark stellar remnant to be discovered, wandering the galaxy, unaccompanied by another star,” Lam said.
The study was published in The Astrophysical Journal and is available on arXiv.
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