A “free” black hole, decoupled from any star, is confirmed by a new study

Two weeks ago Digital appearance reported that astronomers at the University of California, Berkeley believe they have found what may be the first roaming black hole in history. Now another team of scientists from the same institution, conducting a separate, independent analysis, has come to almost the same result, adding weight to the idea that we have identified a free black hole – or something similar – roaming around The Milky Way.

A “parasitic” black hole has been confirmed by two studies. Photo: NASA / SWIFT / AURORE SIMONNET, SONOMA STATE UNIV.

Led by astronomers Casey Lam and Jessica Lu, however, the new work has a “small” difference in conclusion from the previous one. According to the new approach, given the mass range of the object, it may be a neutron star rather than a black hole.

Either way, it means we may have a new tool to search for compact ‘dark’ objects that cannot be detected in our galaxy by measuring how their gravitational fields bend and distort light from distant stars as they pass them, a technique called gravity microlensing. “This is the first free-floating black hole or neutron star discovered with a gravitational micro-lens,” Lu said.

Black hole or neutron star? In the end, just as much

“With this method, we are able to probe these solitary and compact objects and weigh them. I think we opened a new window for the dark objects that can not be seen otherwise.”

Black holes are believed to be the collapsed nuclei of massive stars that have reached the end of their lives and expelled their outer material. These black hole precursor stars – larger than 30 times the mass of the Sun – are thought to have a relatively short lifespan.

According to the best estimates, there should be between 10 million and 1 billion black holes with star mass in space that drift silently through the galaxy.

But since they emit no light that we can detect, they are almost impossible to observe. Unless substance falls into it, a process that generates x-rays in the space around the black hole. Otherwise, if a black hole just moves around without doing anything, we have almost no chance of detecting it.

Almost. Two separate gravitational microlensing studies – the Optical Gravitational Lensing Experiment (OGLE) and the Observations of Microlensing in Astrophysics (MOA) – independently recorded an event that eventually peaked on July 20, 2021.

This event was named MOA-2011-BLG-191 / OGLE-2011-BLG-0462 (abbreviated OB110462), and because it was unusually long and light, the researchers sought refuge for further investigation.

“The duration of the glow event is a clue as to how much of the foreground lens diverts light from the background star,” says Lam. “Long events are more likely to be attributed to black holes. However, this is not a guarantee because the duration of the glow episode depends not only on the mass of the foreground lens but also on the speed at which the foreground lens and star d background move. in relation to each other. ”.

But according to the scientist, “by also taking measurements of the background position of the background star, we can confirm that the foreground lens is actually a black hole.” In this case, observations of the region have been made eight times using the Hubble Space Telescope up to 2017.

1655475910 939 An all-star free black hole is confirmed.webp
The illustration shows how the Hubble Telescope sees a gravitational microlens event. Photo: NASA, ESA, STScI, Joseph Olmsted

Based on a thorough analysis of these data, a team of astronomers led by Kailash Sahu of the Space Telescope Science Institute concluded that the culprit was a microlensed black hole 7.1 times the mass of the Sun, 5,153 years away. .

Lu and Lam’s analysis now adds more Hubble data, captured in 2021. Their team found that the object is slightly smaller, between 1.6 and 4.4 times the mass of the Sun, meaning the object may be a neutron star. It is also the collapsing core of a massive star, which started between 8 and 30 times the mass of the Sun.

The resulting object is supported by what is called neutron degeneration pressure, whereby 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 have been found below about 5 times the mass of the Sun. This is called the smallest mass gap. If the work of Lam and his colleagues is correct, it means that we can have the detection of an object with low mass deviation in our hands.

The two teams came back with different masses for the objective object because their analyzes gave different results for the relative motions of the compact object and the star lens.

Sahu and his team discovered that the compact object moves at a relatively high speed of 45 kilometers per second after a violent birth: a disorderly supernova explosion can cause the nucleus to collapse at lightning speed.

However, Lam and his colleagues observed 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 present, it is impossible to draw a definite conclusion about OB110462, but astronomers hope to learn much from the discovery of more of these objects in the future. “In any case, the object is the first black remnant to be discovered wandering in the galaxy unaccompanied by another star,” Lam said.

The research has been accepted for publication by The Astrophysical Journal and is available on the arXiv prepress server.

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