They discover a free-floating ‘dark’ black hole in our galaxy

In early 2022, Hubble astronomers, using gravitational microlensing, were able to detect, for the first time, a rogue black hole about 5,000 light-years from Earth. Now, with more precise measurements, scientists have been able to determine an approximate mass of this object. And here comes the surprise. The mass of this black hole is surprisingly low, which means there’s a chance that it might not even be a black hole. What could it be?

Black holes are difficult to detect
Black holes, by their very nature, are difficult objects to detect; they have such powerful gravity that light can escape from them, so we usually detect them by their gravitational influence on other objects or by radiation created by the surrounding matter they’re devouring. Essentially, they are invisible to astronomers.

The rogue object newly detected by observing the brightness of a more distant star as its light was distorted by the object’s strong gravitational field, is located about 5,000 light-years away, in the Carina-Sagittarius spiral arm of our galaxy. . Two large independent international teams now publish their conclusions that, although the results of both differ slightly, the two investigations suggest the presence of a relatively compact object. Because both microlensing studios captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.

What it’s like? What is?
The researchers estimate the invisible compact object’s mass to be between 1.6 and 4.4 times that of the Sun. Because astronomers think the leftover remnant of a dead star must weigh more than 2.2 solar masses to collapse into a black hole, experts warn that the object could be a neutron star rather than a black hole. Neutron stars are also very compact, dense objects, but their gravity is balanced by internal neutron pressure, preventing further collapse into a black hole.

“As much as we’d like to say it’s definitely a black hole, we need to report all allowable solutions. This includes lower-mass black holes and possibly even a neutron star,” the authors explain.

Either way, this object is the first dark stellar remnant, a stellar “ghost”, discovered wandering the galaxy without pairing with another star.

“This is the first free-floating black hole or neutron star discovered with gravitational microlensing. With microlensing, we can probe these lonely, compact objects and weigh them. I think we have opened a new window on these dark objects, which cannot be seen from another way,” explains Jessica Lu, associate professor of astronomy at UC Berkeley and co-author of the paper.

Both teams also estimated the speed of the supercompact lens object. Lu/Lam’s team found a relatively calm speed, less than 30 kilometers per second. The second team, from the Space Telescope Science Institute (STScI), found an unusually high speed, 45 km/s, which they interpreted as the result of an additional ‘kick’ that the supposed black hole received from the supernova that generated it.

Reference: Casey Y. Lam, Jessica R. Lu, Andrzej Udalski, Ian Bond, David P. Bennett, Jan Skowron, Przemek Mroz, Radek Poleski, Takahiro Sumi, et al An isolated mass gap black hole or neutron star detected with astrometric microlensing . Accepted to APJ Letters, 2022

Kailash C. Sahu, Jay Anderson, Stefano Casertano, Howard E. Bond, Andrzej Udalski, Martin Dominik, Annalisa Calamida, Andrea Bellini, Thomas M. Brown, et al. An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing. Accepted to APJ, 2022