A probable “isolated” black hole discovered for the first time

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Using the gravitational microlens method, a team of astronomers led by the University of California, Berkeley, may have discovered the first free-moving black hole, born from the collapse of a massive star. Scientists estimate its mass to be between 1.6 and 4.4 solar masses, suggesting that it may also be a neutron star. Either way, it’s the first time such a “star ghost” has been discovered as it orbits the galaxy.

There are several types of black holes; we distinguish in particular the supermassive black holes, which are at the center of the galaxies, the intermediate black holes, of a few thousand solar masses, and the black holes of the stars, of only a few solar masses, which are formed after the gravitational collapse of a massive star. These black holes are, of course, optically invisible, and astronomers must use indirect observation methods to detect their presence.

These methods are based on the effects of the black hole on its immediate surroundings. Because they are isolated, star-shaped black holes are particularly difficult to detect. However, there are hundreds of millions of them in our galaxy! For the first time, scientists believe they have discovered such a black hole thanks to what is called the gravitational microlens effect: the light from a distant star has been distorted for a relatively long time by the supposed black hole’s powerful gravitational field.

Black hole or neutron star?

Astronomers believe that the remains of a dead star must be heavier than 2.2 solar masses to collapse into a black hole. Therefore, the team behind the discovery points out that it may also be a neutron star and not a black hole. Neutron stars, which are also caused by the collapse of massive stars, are dense and compact objects; the difference is that their gravity is compensated by the internal pressure of the neutrons, which prevents them from turning into black holes.

It is at least the first time that such an object, independent of any other star, has been discovered. ” It is the first free-floating black hole or neutron star discovered by gravitational microlensing. […] I think we’ve opened a new window on these dark objects that can not be seen otherwise said Jessica Lu, associate professor of astronomy at UC Berkeley.

Until now, star-sized black holes had only been detected in binary systems. In this case, their presence can be detected thanks to X-rays – produced when the substance from the neighboring star is absorbed by the black hole – or via gravitational wave detectors, produced by the collision of two black holes.

For scientists, it is important to determine how many of these dark, compact objects are in the Milky Way to better understand the evolution of stars and our galaxy. Researchers also hope to find primordial black holes among them – hypothetical micro-black holes that would have formed during the Big Bang.

In the paper presenting their discovery, Lu and his collaborators present the analysis of five candidate black holes in total, all identified from gravitational microlens studies and astrometric data from the Hubble Space Telescope; they measured the mass and brightness of each lens to determine if it was a black hole or not. They claim that four of them are not black holes (because their masses are less than 2 solar masses), two of which are probably white dwarfs or neutron stars.

Nearly 200 million roaming black holes in the galaxy

Doubt exists for the fifth object, dubbed OB110462. This lens, located between 2280 and 6260 light-years, appeared dark and was therefore not a star; the stars’ brightness lasted almost 300 days, and the distortion of the background star’s position also lasted a long time, which especially challenged Lu and his team. ” The duration of the luminous event is an index of the mass of the foreground lens, which distorts the light from the background star. explained Casey Lam, co-author of the study. According to him, about 40% of microlens events lasting longer than 120 days are likely to be black holes.

The duration of the exposure depends not only on the mass of the foreground lens, but also on the speed at which this lens and the background star move relative to each other. Further observations and modeling will therefore be needed to confirm that it really is a black hole.

Based on these five gravitational lenses, the team further concluded that the probable population of isolated black holes in the galaxy is 200 million, which is roughly in line with the predictions of most theorists (who estimate that their abundance is between 10 million and 1 billion).

Namely, that another team, from the Space Telescope Science Institute (STScI) in Baltimore, investigated the same microlens event from the same photometric and astrometric data: it in turn confirms that the object’s mass is closer to 7.1 solar masses. As a result, these researchers believe it is undoubtedly a black hole that they believe is about 5,153 light-years away. Lu and Lam believe that these discrepancies in the results obtained are due to the fact that astrometric and photometric data provide different measurements of the relative movements of foreground and background objects.

Both teams also estimated the object’s speed: Lus’ team found a speed of less than 30 km / s, while the STScI team found an unusually high speed, 45 km / s, as she interpreted it as a result of an extra “boost” “it alleged black holes received from the supernova that generated it. Conversely, the lower velocity calculated by Lu could support a new theory that black holes are not the result of supernovae, but rather “failed” supernovae.

Source: C. Lam et al., ArXiv

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