The first detection of what appears to be a rogue black hole drifting through the Milky Way, revealed earlier this year, has just received significant validation.
A second team of scientists, conducting a separate and independent analysis, came to almost the same conclusion, adding weight to the idea that we have potentially identified a rogue black hole roaming the galaxy.
Led by astronomers Casey Lam and Jessica Lu of the University of California, Berkeley, the new work came to a slightly different conclusion, however. Given the object’s mass range, it could be a neutron star rather than a black hole, according to the new study.
Either way, this means we could have a new tool to search for compact “dark” objects that would otherwise be undetectable in our galaxy, by measuring how their gravitational fields warp and distort starlight. distant as they pass in front of them, called the gravitational microlens.
“This is the first free-floating black hole or neutron star discovered with a gravitational microlens,” Lu said.
“With the microlens, we are able to probe these solitary, compact objects and weigh them. I think we have opened a new window into these dark objects, which otherwise cannot be seen.”
Black holes are theorized to be the collapsed cores of massive stars that have reached the end of their lives and ejected their outer material. These black hole precursor stars – larger than 30 times the mass of the Sun – are thought to have relatively short lives.
According to our best estimates, there should therefore be as many as 10 million to 1 billion stellar-mass black holes, drifting peacefully and silently through the galaxy.
But black holes are called black holes for a reason. They emit no 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 is lying around, doing nothing, we have almost no way of detecting it.
Almost. What a 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 we might see a distant star appear brighter and in a different position than it normally appears.
On June 2, 2011, that is exactly what happened. Two separate surveys of microlensing – the 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 moved on for further examination.
“The duration of the brightening event is an index of the mass of the foreground lens that bends the light from the background star,” Lam explains.
“Long events are more likely due to black holes. This is not a guarantee, however, because the duration of the brightening episode depends not only on the mass of the foreground lens, but also on the the speed at which the foreground lens and the background star move 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 region were taken eight times using the Hubble Space Telescope, up to 2017.
From an in-depth analysis of this data, a team of astronomers led by Kailash Sahu of the Space Telescope Science Institute concluded that the culprit was a microlensing black hole clocked at 7.1 times the mass of the Sun, at a distance 5,153 light-years away.
Lu and Lam’s analysis now adds more data from Hubble, as recently captured as 2021. Their team found the object is quite a bit smaller, between 1.6 and 4.4 times the mass of the Sun. .
This means the object could be a neutron star. It’s also the collapsed core of a massive star, one that started out between 8 and 30 times the mass of the Sun.
The resulting object is supported by what is called neutron degeneracy pressure, whereby 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 limit of about 2.4 times the mass of the Sun.
Interestingly, no black holes have been detected below about 5 times the mass of the Sun. This is called the lower mass gap. If Lam and his colleagues’ work is correct, that means we could have the detection of a low-mass-gap object in our hands, which is very tantalizing.
The two teams came back with different masses for the lens object because their analyzes gave different results for the relative motions of the compact object and the lens star.
Sahu and his team discovered that the compact object was moving at a relatively high speed of 45 kilometers per second, following a natal kick: an unbalanced supernova explosion can send the core collapsing at full speed.
Lam and his colleagues, however, traveled 30 kilometers per second. This result, they say, suggests that a supernova explosion may not be necessary for the birth of a black hole.
At this time, it’s impossible to draw a firm conclusion from OB110462 about the correct estimate, but astronomers expect to learn a lot from the discovery of more of these objects in the future.
“Anyway, the object is the first dark stellar remnant discovered wandering the galaxy unaccompanied by another star,” Lam said.
The search was accepted in The Astrophysical Journaland is available on arXiv.