The first detection of what appears to be a rogue black hole drifting through the Milky Way, revealed earlier this year, has been validated.
A second team of scientists, who conducted a separate and independent analysis, came to almost the same conclusion, adding weight to the idea that we’ve identified a rogue black hole roaming the galaxy.
Led by astronomers Cassie Lamm and Jessica Lu of the University of California, Berkeley, the new work came to a slightly different conclusion. Depending on the body’s mass range, it could be a neutron star, not a black hole, according to the new study.
Either way, this means we may have a new tool to search for undetectable “dark” compact objects in our galaxy, by measuring the way gravitational fields twist and distort the light of distant stars as it passes in front of them, called gravitational microgravity.
“This is the first floating black hole or neutron star to be detected using a gravitational microlensing,” Lu says.
“By using the finer lens, we can examine and weigh these isolated, compressed objects. I think we’ve opened a new window on these dark objects, which cannot be seen any other way.”
Black holes are assumed to be the collapsed cores of massive stars that have reached the end of their lives and expelled their outer matter. Such black hole starting stars – thirty times the mass of the Sun – are believed to live relatively short lives.
According to our best estimates, there should be as many as 10 million to a billion stellar-mass black holes, drifting peacefully and quietly through the galaxy.
But black holes are called black holes for a reason. It does not emit any light that we can detect, unless material falls on it, a process that generates X-rays from space around the black hole. So, if a black hole is just hanging around, doing nothing, there’s almost no way to detect it.
approx. What a black hole has is an intense gravitational field, so strong that it distorts any light that passes through it. For us, as observers, this means that we may see a distant star that appears brighter, and in a different position, than it normally appears.
On June 2, 2011, that’s exactly what happened. Two separate microlens surveys – the Optical Gravitational Lens Experiment (OGLE) and the Microlensing Observation in Astrophysics (MOA) – independently recorded an event that culminated on July 20.
The event was named MOA-2011-BLG-191/OGLE-2011-BLG-0462 (abbreviated to OB110462), and because it was unusually long and unusually bright, it invited scientists to take a closer look.
“How long the bright event lasts is an indication of how large the foreground lens bends the background star’s light,” Lamm explains.
“Longer events are most likely due to black holes. This is not a guarantee, because the duration of the bright ring depends not only on how massive the foreground lens is, but also on how quickly the foreground lens and background star are moving relative to each other.”
“However, by also obtaining measurements of the apparent position of the background star, we can confirm whether the foreground lens is really a black hole.”
In this case, observations of the area were taken on eight separate occasions with the Hubble Space Telescope, up to 2017.
From a deep analysis of this data, a team of astronomers led by Kailash Sahu of the Space Telescope Science Institute concluded that the culprit was an infinitesimal black hole 7.1 times the mass of the Sun, 5,153 light-years away.
Low and Lam’s analysis now adds more data from Hubble, captured as recently as 2021. Their team found the object to be somewhat smaller, between 1.6 and 4.4 times the mass of the Sun.
This means that the object could be a neutron star. This is also the collapsed core of a massive star, whose mass began between 8 and 30 times the mass of the Sun.
The resulting object is supported by something called neutron degeneration pressure, in which the neutrons do not want to occupy the same space; This prevents it from completely collapsing into a black hole. Such an object has a mass of about 2.4 times the mass of the Sun.
In addition, no black holes less than 5 times the mass of the Sun have been detected. This is referred to as the bottom block gap. If Lamm and her colleagues’ work is correct, this means that we can detect a lower mass gap object in our hands, which is very puzzling.
The two teams returned with different masses of the lensing body because they analyzed different results for the relative motions of the compressed object and the lensed star.
Saho and his team discovered that the compact object was moving at a relatively high speed of 45 kilometers per second, as a result of a natal kick: an unbalanced supernova explosion that could send a rapidly collapsing core away.
But Lamm and her colleagues got 30 kilometers per second. This result, they say, suggests that a supernova explosion may not have been necessary for the birth of a black hole.
At the moment, it is impossible to draw a definite conclusion from OB110462 about the correct estimate, but astronomers expect to learn a lot from the discovery of more such objects in the future.
“Whatever this object is, it is the first dark stellar remnant to be discovered wandering through the galaxy unaccompanied by another star,” Lamm says.
The search has been accepted Astrophysical Journalavailable at arXiv.