image: an image of the Hubble Space Telescope of a distant star that is illuminated and distorted by an invisible but very compact and heavy object between it and the Earth. The compact object – estimated by astronomers at UC Berkeley to be between 1.6 and 4.4 times the mass of our sun – could be a free-floating black hole, one of perhaps 200 million in the Milky Way galaxy. See more
Credit: Image courtesy of STScI / NASA / ESA
If, according to astronomers, the death of the big stars left behind black holes, there should be hundreds of millions of them scattered throughout the Milky Way galaxy. The problem is that the isolated black holes are invisible.
Now, a team led by the University of California, Berkeley, astronomers have first discovered what a free-floating black hole could be by observing the illumination of a more distant star as its light is distorted by the object’s strong gravitational field. gravitational microleading.
The team, led by graduate student Casey Lam and Jessica Lou, an 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 that of the sun. Because astronomers believe the remains of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, UC Berkeley researchers warn that the object could be a neutron star instead of a black hole. Neutron stars are also dense, highly compact objects, but their gravity is balanced by internal neutron pressure, which prevents further collapse to 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 the galaxy, unpaired with another star.
“This is the first free-floating black hole or neutron star detected by gravitational microlensing,” Lou said. “With microlensing, we can explore these lonely, compact objects and weigh them. I think we’ve opened a new window to these dark objects that can’t be seen otherwise.”
Determining how many of these compact objects inhabit the Milky Way galaxy will help astronomers understand the evolution of stars – especially how they die – and our galaxy, and perhaps reveal whether any of the invisible black holes are primary black holes. which some cosmologists believe were produced in large quantities during the Big Bang.
The analysis of Lam, Lou and their international team has been accepted for publication in The Astrophysical Journal Letters. The analysis included four other microlens events that the team concluded were not caused by a black hole, although two were probably caused by a white dwarf or a neutron star. The team also concluded that the probable population of black holes in the galaxy is 200 million – about what most theorists predict.
Same data, different conclusions
In particular, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same event with microlensing and claimed that the mass of the compact object was closer to 7.1 solar masses and undoubtedly a black hole. A paper describing the analysis by the STScI team, led by Kailash Sahu, has been accepted for publication in The Astrophysical Journal.
Both teams used the same data: photometric measurements of the distant star’s illumination because its light was distorted or a “lens” from the supercompact object, and astrometric measurements of the distant star’s location shift in the sky due to gravity distortion from the lens. Photometric data came from two microlens studies: the OGLE Optical Gravity Lens Experiment (OGLE), which uses a 1.3-meter telescope in Chile operated by the University of Warsaw, and the Microlensing Observations in Astrophysics (MOA) experiment, which was mounted on 1, An 8-meter telescope in New Zealand, operated by the University of Osaka. Astrometric data comes from NASA’s Hubble Space Telescope. STScI manages the telescope’s scientific program and conducts its scientific operations.
Because both microlens have captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.
While studies like these have found about 2,000 stars illuminated by microlens each year in the Milky Way galaxy, the addition of astrometric data is what allows both teams to determine the mass of the compact object and its distance from Earth. The team, led by the University of California, Berkeley, estimated it to be between 2,280 and 6,260 light-years (700-1920 parsecs) toward the center of the Milky Way galaxy and near the large bulge that surrounds the galaxy’s central massive black hole. .
The STScI group estimates that it is about 5,153 light-years (1,580 parsecs) of us.
I’m looking for a needle in a haystack
Lou and Lam first became interested in the site in 2020, after the STScI team tentatively concluded that five microlens events observed by Hubble – all of which lasted more than 100 days and could therefore be black holes – may not be caused by compact objects after all.
Lou, who has been searching for free-floating black holes since 2008, said the data would help her better estimate their abundance in the galaxy, which is estimated at between 10 million and 1 billion. To date, star-sized black holes have been discovered only as part of binary star systems. Black holes in binary systems are seen either in X-rays obtained when material from the star falls on the black hole, or from recent gravitational wave detectors that are sensitive to the fusion of two or more black holes. But these events are rare.
“Casey and I saw the data and we were really interested. We said, “Wow, no black holes. “It’s amazing, even though there should have been,” Lou said. “So we started looking at the data. If there really were no black holes in the data, then that wouldn’t fit our model of how many black holes there should be in the Milky Way. Something should change in our understanding of black holes. – or their number, or how fast they move, or their masses. “
When Lam analyzed photometry and astrometry for the five microlensing events, she was surprised that one, OB110462, had the characteristics of a compact object: the lens object looked dark and therefore not a star; stellar enlightenment lasted a long time, nearly 300 days; and the distortion of the background star’s position was also long-lasting.
The length of the lens event was the main signal, Lam said. In 2020, she showed that the best way to look for microlens for black holes is to look for very long events. Only 1% of detectable events with microlenses are probably from black holes, she said, so looking at all the events would be like looking for a needle in a haystack. But, Lam estimated, about 40% of microlensing events that last more than 120 days are probably black holes.
“How long the lightening lasts is a hint at how massive the lens in the foreground is, which bends the light of the background star,” Lam said. “Long events are more likely due to black holes. However, this is not a guarantee, as the duration of the lighting episode depends not only on how massive the foreground lens is, but also on how fast the foreground lens and the background star move. To each other. However, by also obtaining measurements of the visible position of the background star, we can confirm whether the lens in the foreground is indeed a black hole. “
According to Lou, the gravitational influence of OB110462 on the light of the background star is incredibly long. It took about a year for the star to shine to its peak in 2011, then about a year to return to normal.
More data will distinguish the black hole from the neutron star
To confirm that OB110462 was caused by a supercompact object, Lou and Lam requested more astrometric data from Hubble, some of which arrived last October. These new data show that the change in the position of the star as a result of the gravitational field of the lens is still observed 10 years after the event. Further Hubble observations of microlenses are tentatively planned for the fall of 2022.
Analysis of the new data confirmed that OB110462 was probably a black hole or a neutron star.
Lou and Lam suspect that the different conclusions of the two teams are due to the fact that astrometric and photometric data provide different measures for the relative movements of objects in the foreground and background. Astrometric analysis also differs between the two teams. The UC-led Berkeley team says it is not yet possible to tell if the object is a black hole or a neutron star, but hopes to resolve the discrepancy with more Hubble data and improved analysis in the future.
“As much as we want to say that this is definitely a black hole, we need to report all the solutions allowed. This includes both black holes with lower mass and probably even a neutron star,” Lou said.
“If you can’t believe the curve of light, the brightness, then that says something important. If you don’t believe the position of time, it tells you something important,” Lam said. “So, if one of them is wrong, we need to understand why. Or the other possibility is that what we measure in both data sets is correct, but our model is incorrect. Photometric and astrometric data are derived from the same physical process, which means that brightness and position must be consistent with each other. So there’s something missing. “
Both teams also appreciated the speed of the super-compact lens. The Lu / Lam team found …
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