If the death of large stars leaves behind black holes, as astronomers believe, there should be hundreds of millions of them scattered throughout the Milky Way. The problem is that isolated black holes are not visible.

Now, a team led by the University of California, Berkeley, astronomers have discovered for the first time what could be a free-floating black hole by observing the brightness of a distant star as its light is distorted by an object’s strong gravitational field – therefore – called microgravity.

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

Whether it’s a black hole or a neutron star, the object is the first dark stellar remnant – a stellar “ghost” – discovered wandering through the galaxy unassociated with another star.

“This is the first floating black hole or neutron star to be detected by microgravitational lenses,” Lu said. “By using the finer lens, we can examine and weigh these isolated, compressed objects. I think we’ve opened a new window on these dark objects, which cannot be seen any other way.”

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, possibly revealing whether any of the unseen black holes are primordial black holes, which he considers Some cosmologists believe that large quantities were produced during the Big Bang.

The analysis by Lam, Lu and their international team has been accepted for publication in Astrophysical Journal Letters. 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 likely number of black holes in the galaxy is 200 million – about what most theorists had expected.

Same data, different conclusions

Notably, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and claimed that the mass of the compact object is closer to 7.1 solar masses and an undisputed black hole. Paper describing analysis by the STScI team led by Kailash Sahuhas been accepted for publication in Astrophysical Journal.

Both teams used the same data: photometric measurements of a distant star’s brightness as its light was distorted or “reflected” by the highly compressed object, and astronomical measurements of the distant star’s changing position in the sky as a result of gravity. distortion by the lens object. The optical data came from two microlens surveys: the Optical Gravitational Lens Experiment (OGLE), which uses a 1.3-meter telescope in Chile operated by the University of Warsaw, and the Microlens observations in Astrophysics (MOA), which is mounted on a 1.8-meter telescope in New Zealand operated by the University of Warsaw. Osaka University. Astronomical data came from NASA’s Hubble Space Telescope. STScI manages the telescope’s science program and conducts its science operations.

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

While surveys like this one discover about 2,000 bright stars by microlensing each year in the Milky Way, it was the addition of astronomical data that allowed the two teams to determine the compact object’s mass and distance from Earth. The team led by the University of California, Berkeley, estimated that it is located between 2,280 and 6260 light-years away (700-1920 parsecs), toward the center of the Milky Way and near the large bulge that surrounds the galaxy’s central supermassive black hole.

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

I’m looking for a needle in a haystack

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

Lu, who has been searching for free-moving black holes since 2008, thought the data would help her better estimate their abundance in the galaxy, which was roughly estimated to be between 10 million and 1 billion. So far, black holes the size of stars have only been found as part of binary star systems. Black holes are seen in binaries either in X-rays, which are produced when material from a star falls onto a black hole, or by modern gravitational wave detectors, which are sensitive to the mergers of two or more black holes. But these events are rare.

“Casey and I watched the data and got really interested. We said, ‘Wow, there are no black holes,'” Lu said. That’s amazing, “even though it should have been there.” “And so, we started looking at the data. If there really weren’t black holes in the data, this wouldn’t match our model of how many black holes should be in the Milky Way. Something had to change in the understanding of black holes—either their number, speed, or mass.”

When Lamm analyzed the photometric and astrometry of the five minute lens events, I was surprised that one, OB110462, had the characteristics of a compact body: the lens body appeared dark, and thus not a star; stellar brightness lasted for a long time, almost 300 days; The distortion of the background star’s position was also long-term.

Lamm said the length of the lens event was the main tip. In 2020, it showed that the best way to search for black hole microlenses is to look for very long events. Only 1% of the minute lens events that can be detected are likely from black holes, she said, 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 lasting more than 120 days are likely to be black holes.

“How long the bright event lasts is a hint at how massive the foreground lens bends the background star’s light,” Lamm said. “Longer events are most likely due to black holes. This is not a guarantee, because the duration of the bright ring depends not only on how massive the foreground lens is, but also on how quickly the foreground lens and background star are moving relative to each other. However, by also obtaining measurements For the apparent position of the background star, we can confirm whether the foreground lens is really a black hole.”

According to Lu, the gravitational effect of OB110462 on the background star’s light was surprisingly long. It took about a year for the star to light up to its peak in 2011, and then about a year to get back to normal.

More data will distinguish a black hole from a neutron star

To confirm that OB110462 resulted from an extremely compact object, Low and Lam requested more astronomical data from Hubble, some of which arrived last October. This new data showed that the change in the star’s position due to the lens’ gravitational field could still be observed 10 years after the event. More Hubble observations of microlensing are tentatively scheduled for the fall of 2022.

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

Low and Lam suspect that the two teams’ different conclusions are due to the fact that the astronomical and photometric data give different measures of the relative motions of the fore and aft objects. Astrological analysis also differs between the two teams. The UC Berkeley team argues that it is not yet possible to distinguish whether the object is a black hole or a neutron star, but they hope to resolve the discrepancy with more Hubble data and improved analysis in the future.

“As much as we would definitively say it’s a black hole, we should report all permissible solutions,” Lu said. “This includes both black holes of lower mass and perhaps even a neutron star.”

“If you can’t believe the curve of the light, the brightness, that means something important. If you can’t believe the situation versus time, that tells you something important,” Lamm said. “So, if one of them is wrong, we have to understand why. Or another possibility is that what we measure in the two data sets is correct, but our model is incorrect. The photometric and astrometric data originate from the same physical process, which means that brightness and position must be consistent. With each other. So, something is missing there.”

Both groups also estimated the velocity of the ultrafine lens body. The Lu/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 kick that the so-called black hole got from the supernova it generated.

Low interprets her team’s low velocity estimate as possible support for a new theory that black holes are not the result of supernovae – the prevailing assumption today – but instead come from failed supernovae that don’t make a bright splash in the universe or give the resulting black hole a kick.

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