If the death of large stars leaves black holes, as astronomers believe, there must be hundreds of millions scattered throughout the Milky Way. The problem is that isolated black holes are invisible.

Now, a team of astronomers led by the University of California at Berkeley have discovered for the first time what a hovering black hole could be by watching the glow of a distant star as its light is distorted by the object’s strong gravitational field – hence – called microgravity.

The team is led by graduate student Casey Lam and Jessica LoweAn associate professor of astronomy at the University of California at Berkeley estimates the mass of a compact invisible object to be 1.6 to 4.4 times the mass of the Sun. Because astronomers believe that the remnants of a dead star would have to be heavier than 2.2 solar masses to collapse into a black hole, researchers at the University of California at Berkeley warn that the object could be a neutron star rather than a neutron star black hole. Neutron stars are also very dense and compact objects, but their gravity is balanced by the internal pressure of neutrons, which prevents further collapse into a black hole.

Whether it’s a black hole or a neutron star, this object is the first remnant of a dark star – a stellar “ghost” found in the galaxy, unrelated to another star.

“This is the first hovering black hole or neutron star detected with a microgravity lens,” Lu said. “Using the thinnest lenses, we can examine and weigh these isolated, compressed objects. I think we’ve opened a new window for these dark objects that can’t be seen otherwise.”

Determining how many of these compact objects inhabit the Milky Way will help astronomers understand the evolution of stars – in particular, how they die – and the evolution of our galaxy, perhaps revealing whether any of the invisible black holes are primordial black holes. believes Some cosmologists believe that a large amount was produced during the Big Bang.

The analysis by Lam, Lu and their international team has been accepted for publication in Letters from the astrophysical newspaper. The analysis includes four other microlensing events that the team concluded were not caused by a black hole, although two are likely caused by a white dwarf or a neutron star. The team also concluded that the probable number of black holes in the galaxy is 200 million – more or less what most theorists expected.

Same data, different conclusions

Remarkably, a rival team at the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and declared that the mass of the compact object is closer to 7.1 solar masses and is an indisputable black hole. Article describing the analysis of the STScI group led by Kailash Sahuaccepted for publication in Astrophysical journal.

Both teams used the same data: photometric measurements of the brightness of a distant star when its light was distorted or “reflected” by a highly compressed object, and astronomical measurements of the change in the position of a distant star in the sky under the influence of gravity. lens distortion. The optical data was obtained from two microlensing studies: the Optical Gravitational Lens Experiment (OGLE), which uses the 1.3-meter telescope of the University of Warsaw in Chile, and observations using Microlenses in Astrophysics (MOA), which is set to 1.8 meter telescope. meter telescope. The two-meter telescope in New Zealand is owned by the Warsaw University of Osaka University. The astronomical data came from NASA’s Hubble Space Telescope. The STScI manages the telescope’s science program and conducts its science operations.

Since the high-precision lens recognition captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.

While studies like this one find about 2,000 bright microlens stars each year in the Milky Way, it was the addition of astronomical data that allowed the two teams to determine the mass and distance of the compact object from Earth. A team led by the University of California at Berkeley has calculated that it is located between 2280 and 6260 light-years (700-1920 parsecs), towards the center of the Milky Way and not far from the large bulge surrounding the central supermassive black galaxy. Milky Way galaxy. hole.

The STScI cluster is estimated to be about 5,153 light-years (1,580 parsecs) away.

looking for a needle in a haystack

Lu and Lam first became interested in the body in 2020 after the STScI team initially concluded that Five microlensing events The ones that Hubble observed – all of which lasted more than 100 days and therefore could be black holes – are most likely not caused by compact objects.

Lu, who has been searching for floating black holes since 2008, says the data will help her better estimate the number of black holes in the galaxy, which is estimated to be between 10 million and 1 billion. So far, star-sized black holes have only been found in binary star systems. Black holes in binary systems are visible either in X-rays, which are produced when star matter falls into a black hole, or in modern gravitational wave detectors, which are sensitive to the merger of two or more black holes. But such events are rare.

“Casey and I looked at the data and were very interested. We said, “Wow, black holes don’t exist,” Lou said. It’s amazing “even though it should have been there”. “And then we started looking at the data. If there really were no black holes in the data, this would not fit our model of how many black holes there should be in the Milky Way. Something had to change in the understanding of black holes, be it their number, speed or mass.”

When Lam analyzed the photometry and astrometry of five-minute lens events, I was surprised that one of them, OB110462, had the characteristics of a compact body: the lens body looked dark and therefore not a star; the glow of the star lasted for a long time, almost 300 days; The background star position distortion was also long-term.

Lamm said the length of the lens event was a major clue. In 2020, he showed that the best way to look for black hole microlensing is to look for very long events. Only 1% of the tiny lensing events that can be detected come from black holes, she says, so looking at all the events would be like looking for a needle in a haystack. But, according to Lamm, about 40% of microlensing events that last longer than 120 days are probably black holes.

“The duration of a bright event is key to understanding how massive a lens in the foreground refracts the light of a star in the background,” Lamm said. “Longer events are likely associated with black holes. This is not a guarantee, because the duration of the bright ring depends not only on the mass of the foreground lens, but also on how fast the foreground lens and background star are moving relative to the measurements for the apparent position of the background star, we can confirm that the foreground lens is indeed a black hole.

According to Lou, OB110462’s gravitational impact on the background starlight was surprisingly long. It took the star about a year to reach its peak in 2011, and then about another year to return to normal.

More data could help distinguish a black hole from a neutron star

To confirm that OB110462 is the result of an extremely compact object, Lowe and Lam requested additional astronomical data from Hubble, some of which were received last October. These new data showed that the change in the position of the star due to the gravitational field of the lens can still be observed 10 years after the event. Additional Hubble observations of microlensing are planned for autumn 2022.

Analysis of the new data confirmed that OB110462 was likely a black hole or a neutron star.

Lowe and Lam suspect that the different conclusions of the two groups are due to the fact that astronomical and photometric data provide different measurements of the relative motion of forward and backward objects. Astrological analysis also differs between the two teams. The UC Berkeley team says it’s not yet possible to distinguish whether an object is a black hole or a neutron star, but they hope to close the discrepancy with more Hubble data and better analysis in the future.

“As far as we’re definitely saying it’s a black hole, we have to report all valid solutions,” Lu said. “This includes both lower-mass black holes and possibly even a neutron star.”

“If you can’t believe the light curve, the brightness, it means something important. If you can’t believe in the dependence of the situation on time, it tells you something important, ”said Lamm. “So if one of them is wrong, we need to understand why. Or another possibility is that what we measured on both datasets is correct, but our model is incorrect. Photometric and astrometric data result from the same physical process, meaning that brightness and position must be consistent. Together. So something is missing there.

Both groups also appreciated the speed of the ultra-thin lens barrel. The Lou/Lam team found a relatively moderate speed, less than 30 kilometers per second. The STScI team found an unusually high speed, 45 km/s, which they interpreted as the result of an extra push that the so-called black hole received from the supernova it spawned.

Lowe interprets his team’s low velocity estimate as possible support for a new theory that black holes are not the result of supernova explosions (this is the prevailing assumption today), but rather failed supernovae that do not produce a bright burst in the universe and do not give the black hole the result of an impact.

Lu and Lam’s work is supported by the National Science Foundation (1909641) and the National Aeronautics and Space Administration (NNG16PJ26C, NASA FINNESS 80NSSC21K2043).