If the death of large stars leaves black holes behind, as astronomers believe, there must be hundreds of millions of black holes in the Milky Way. The problem is that isolated black holes are invisible.
Now, a team led by the University of California, Berkeley, astronomers have discovered for the first time what a floating black hole might be by looking at the glow of a distant star, as its light is distorted by the object’s strong gravitational field – hence the name microgravity.
The team, led by graduate student Casey Lam and UC Berkeley associate professor of astronomy Jessica Low, estimated the dense, invisible object to be between 1.6 and 4.4 times the mass of the sun. Since astronomers believe the dead star’s remnant must be heavier than 2.2 solar masses to collapse into a black hole, UC Berkeley researchers warn that the object may be a neutron star, not a neutron star, a black hole. Neutron stars are also very dense and dense objects, but their gravity is balanced by the internal pressure of the neutrons, preventing further collapse into a black hole.
Whether a black hole or a neutron star, the object is the first dark stellar remnant in the Milky Way unrelated to another star — a kind of stellar “ghost.”
“This is the first floating black hole or neutron star detected by microlensing,” Lu said. “By using the thinnest possible lens, we can inspect and weigh these isolated, compressed objects. I think we’re opening a new window into these dark objects that can’t be seen any other way.”
Determining how many of these compact objects are in the Milky Way will help astronomers understand the evolution of stars—especially how they die—and the evolution of our galaxy, potentially revealing whether these invisible black holes are primordial. It is thought that some cosmologists believe that a large amount was produced during the Big Bang.
Analysis by Lam, Lu and their international team has been accepted for publication in Letters from Astrophysical Journals. The analysis included four other microlensing events, which the team concluded were not caused by black holes, although two could be caused by white dwarfs or neutron stars. The team also concluded that there may be 200 million black holes in the Milky Way — more or less as most theorists expected.
Same data, different conclusions
Notably, a competing team at the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and claimed that the compact object had a mass closer to 7.1 solar masses and an undisputed black hole.The article describing the analysis by the STScI team led by Kailash Sahu was accepted for publication in astrophysical journal.
Both teams used the same data: photometric measurements of the brightness of distant stars when their light is distorted, or “reflected,” by highly compressed objects, as well as measurements of distant stars’ positional changes in the sky due to gravity. Astronomical measurements. Distortion of lens objects. Optical data came from two microlensing surveys: the Optical Gravitational Lensing Experiment (OGLE), which uses the 1.3-meter telescope operated by the University of Warsaw in Chile, and the 1.8-meter telescope installed. New Zealand’s two-meter telescope is operated by the University of Warsaw and Osaka University. Astronomical data from NASA’s Hubble Space Telescope. STScI manages the telescope’s science program and conducts science operations.
Because precision lens recognition captures 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 find about 2,000 microlensed bright stars in the Milky Way each year, it was the addition of astronomical data that allowed the two teams to determine the dense object’s mass and distance from Earth. The team, led by the University of California, Berkeley, estimates that it lies between 2,280 and 6,260 light-years (700-1920 parsecs), close to the center of the Milky Way, close to the large bulge that surrounds the central supermassive black galaxy. Hole.
The STScI cluster is 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 object in 2020 after the STScI team initially concluded that the five microlensing events observed by Hubble — all of which lasted more than 100 days and were therefore likely black holes — were very likely to be black holes. Probably not caused by dense objects.
Lu, who has been searching for floating black holes since 2008, believes the data will help her better estimate their abundance in the Milky Way, which is estimated to be between 10 million and 1 billion. So far, star-sized black holes have only been discovered as part of binary star systems. Black holes are seen in binary stars, or in X-rays, produced when material from a star falls into the black hole, or in modern gravitational-wave detectors, the merger of two or more black holes very sensitive. But such incidents are rare.
“Casey and I looked at the data and were very interested. We said, ‘Wow, there are no black holes,'” Lu said. It’s amazing, “even though it’s supposed to be there”. “Then we started looking at the data. If there are really no black holes in the data, that doesn’t match our model of how many black holes should be in the Milky Way. There has to be a change in the understanding of black holes—whether in terms of quantity, velocity, or mass.”
When Ram analyzed the photometric and astrometrics of the five-minute event of the lens, I was surprised that one of them, the OB110462, has the characteristics of a compact body: the lens body looks dim, so it is not a star; the starlight lasts for a long time, Nearly 300 days; the distortion of the background star position is also long-term.
The duration of the footage incident was the main clue, Ram said. In 2020, it showed that the best way to look for black hole microlenses is to look for very long events. Only 1 percent of the tiny lensing events that can be detected are likely to come from black holes, so looking at them all is like looking for a needle in a haystack, she said. But according to Lamm, about 40 percent of microlensing events lasting longer than 120 days are likely to be black holes.
“The duration of the bright event is a clue to how much the foreground lens bends the background stellar light,” Ram said. “Longer events may be due to black holes. This is not a guarantee, since 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 the background star are moving relative to the measured value of the apparent position of the background star, We can confirm that the foreground shot is indeed a black hole.
According to Lu, the gravitational effect of OB110462 on the background starlight is surprisingly long. The star took about a year to peak in 2011, and then about a year to return to normal.
More data will distinguish black holes from neutron stars
To confirm that OB110462 was produced by an extremely compact object, Low and Lam asked Hubble for more astronomical data, some of which arrived last October. These new data show that 10 years after the event, changes in the stellar position caused by the gravitational field of the lens can still be observed. More Hubble observations of the microlens are planned for fall 2022.
Analysis of the new data confirmed that OB110462 is likely a black hole or a neutron star.
Low and Lam suspect that the two teams’ different conclusions are due to the fact that astronomical and photometric data provide different measurements of the relative motion of front and rear objects. The astrological analysis of the two teams is also different. The UC Berkeley team believes that it is not yet possible to tell whether the object is a black hole or a neutron star, but they hope to resolve this discrepancy in the future with more Hubble data and improved analysis.
“Although we definitely say it’s a black hole, we should report all allowed solutions,” Lu said. “This includes lower-mass black holes and possibly even neutron stars.”
“If you can’t trust the curve and brightness of light, that means something important. If you can’t trust the relationship of a situation to time, that tells you something important,” Ram said. “So if one of them is wrong, we have to understand why. Or another possibility is that our measurements in both datasets are correct, but our model is incorrect. Photometric data and celestial data Derived from the same physical process, which means brightness and position have to be consistent. With each other. So something is missing there.
Both groups also estimated the speed of the ultrathin mirror body. Lu/Lam’s team found a relatively modest speed of less than 30 kilometers per second. The STScI team found an unusually high velocity, 45 km/s, which they interpreted as the result of the extra kick the so-called black hole received from the supernova it spawned.
Low interprets his team’s low-velocity estimates as possible support for a new theory that black holes are not the result of supernovae — the prevailing hypothesis today — but failed supernovae that don’t cause bright splashes in the universe, nor A black hole results in a kick.
Lu and Lam’s work was supported by the National Science Foundation (1909641) and the National Aeronautics and Space Administration (NNG16PJ26C, NASA FINNESS 80NSSC21K2043).
Letter from the Astrophysical Journal
an observational study
Isolated black hole or neutron star with mass gap discovered using astronomical microlensing