(ORDO NEWS) — The first detection of what appears to be a rogue black hole drifting through the Milky Way earlier this year has just received an important confirmation.
A second group of scientists, in a separate, independent analysis, came to almost the same conclusion, adding weight to the idea that we have potentially discovered a rogue black hole roaming the galaxy.
Astronomers Casey Lam and Jessica Lu of the University of California at Berkeley came to a slightly different conclusion. Given the mass range of the object, it may not be a black hole, but a neutron star, the new study says.
In any case, this means we may have a new tool for finding “dark” compact objects otherwise undetectable in our galaxy by measuring how their gravitational fields warp and distort the light of distant stars as they pass in front of them. , which is called gravitational microlensing.
“This is the first free-floating black hole or neutron star detected using gravitational microlensing,” says Lu.
“With microlensing, we can examine these solitary, compact objects and weigh them. I think we’ve opened a new window into these dark objects that can’t be seen in any other way.”
According to the theory, black holes are the collapsed cores of massive stars that have reached the end of their lives and ejected their outer material. Such precursor stars of black holes – more than 30 times the mass of the Sun – are thought to have relatively short lives.
Our best estimate is that there should be between 10 million and 1 billion stellar-mass black holes in the world, drifting peacefully and quietly through the galaxy.
But black holes are called black for a reason. They don’t emit light that we can detect, unless matter falls on them, producing X-rays in the space around the black hole. So if a black hole is just hanging around doing nothing, we have almost no way of detecting it.
Nearly. What a black hole has is an extreme gravitational field so powerful that it bends any light that passes through it. For us observers, this means that we can see a distant star that looks brighter and is in a different position than usual.
This is exactly what happened on June 2, 2011. Two separate microlensing studies – Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA) – independently recorded an event that peaked on July 20.
This event was named MOA-2011-BLG-191/OGLE-2011-BLG-0462 (abbreviated as OB110462), and because it was unusually long and unusually bright, scientists decided to take a closer look at it.
“How long a bright event lasts says a lot about how massive the foreground lens is, bending the background star’s light,” explains Lam.
“Long-lasting events are most likely associated with black holes. However, this is not a guarantee, since the duration of the brightening episode depends not only on how massive the foreground lens is, but also on how fast the foreground lens and the background star are moving relative to each other friend.”
“However, by also obtaining measurements of the apparent position of the background star, we can confirm whether the foreground lens is indeed a black hole.”
In this case, the region was observed eight times with the Hubble Space Telescope until 2017.
After a deep analysis of these data, a team of astronomers led by Kailash Sahu of the Space Telescope Science Institute concluded that the culprit is a microlensing black hole with a mass of 7.1 times that of the Sun and located at a distance of 5,153 light-years from us.
Now Lu and Lam’s analysis has been supplemented with additional data from Hubble taken in 2021. Their team found that the object is somewhat smaller, between 1.6 and 4.4 times the mass of the Sun.
This means that the object could be a neutron star. It is also the collapsed core of a massive star that started out 8 to 30 times the mass of the Sun.
The resulting object is supported by what is called neutron degeneracy pressure, in which neutrons do not want to occupy the same space; this prevents it from completely collapsing into a black hole. The limiting mass of such an object is approximately 2.4 times the mass of the Sun.
Interestingly, not a single black hole has been discovered, the mass of which would be less than the mass of the Sun by about 5 times. This is called the lower mass limit. If the work of Lam and her colleagues is correct, then this means that we can detect an object with a smaller mass gap, which is quite tempting.
Both teams obtained different masses for the lensing object because their analyzes yielded different results on the relative motions of the compact object and the lensed star.
Sahu and his team found that the compact object is moving at a relatively high speed of 45 kilometers per second, which is the result of a natal shock: a supernova explosion can send the collapsed core at high speed.
However, Lam and her colleagues got 30 kilometers per second. This result, they say, suggests that perhaps a supernova explosion is not necessary for the birth of a black hole.
It is currently impossible to draw a firm conclusion on which estimate is correct based on OB110462, but astronomers expect to learn a lot by finding more such objects in the future.
“Whatever it is, the object is the first discovered dark stellar remnant roaming the galaxy unaccompanied by another star,” says Lam.
Contact us: [email protected]