Astronomers may have discovered a "dark" free-floating black hole

Astronomers may have discovered a “dark” free-floating black hole

Micro lensing of compact objects

image: Hubble Space Telescope image of a distant star that became brighter and distorted by an invisible but very compact and heavy object between it and the earth. The compact object – estimated by astronomers from UC Berkeley to be between 1.6 and 4.4 times the mass of our sun – may be a free-floating black hole, one of perhaps 200 million in the Milky Way galaxy.
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Credit: Image courtesy of STScI / NASA / ESA

If, as astronomers believe, the deaths of big stars leave black holes, there should be hundreds of millions of them scattered across the Milky Way galaxy. The problem is that insulated black holes are invisible.

Now a team led by the University of California, Berkeley, astronomers have for the first time discovered what can be a free-floating black hole by observing the brightness of a more distant star when its light was distorted by the object’s strong gravitational field – so-called gravitational micro-lensing.

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

Whether it is a black hole or a neutron star, the object is the first dark star remnant – a star ghost – discovered to have wandered through the galaxy unpaired with another star.

“This is the first free-floating black hole or neutron star discovered with a gravitational micro-lens,” Lu said. “With micro-lensing, we can examine these solitary, compact objects and weigh them. I think we have opened a new window against these dark objects, which can not be seen in any other way.”

Determining how many of these compact objects populate the Milky Way galaxy will help astronomers understand the evolution of stars – especially how they die – and of our galaxy, and perhaps reveal whether any of the invisible black holes are original black holes, which some cosmologists believe were produced in large quantities during the Big Bang.

The analysis by Lam, Lu and their international team has been approved for publication in The Astrophysical Journal Letters. The analysis includes four other microlensing 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 – roughly what most theorists predicted.

Same data, different conclusions

It is noteworthy that a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and claims that the mass of the compact object is closer to 7.1 solar masses and indisputably a black hole. An article describing the analysis of the STScI team, led by Kailash Sahuhas been approved for publication in The Astrophysical Journal.

Both teams used the same data: photometric measurements of the distant star’s luminosity when its light was distorted or “lensed” by the super-compact object, and astrometric measurements of the distant star’s displacement in the sky as a result of gravitational distortion of the objective object. The photometric data came from two microlens studies: the Optical Gravitational Lensing 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 is mounted on a 1 , 8-meter telescope in New Zealand operated by Osaka University. The astrometric data came from NASA’s Hubble Space Telescope. STScI manages the science program for the telescope and conducts its science activities.

Since both microlens surveys captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, abbreviated.

While studies such as these reveal about 2,000 stars illuminated by microlensing each year in the Milky Way galaxy, the addition of astrometric data enabled the two teams to determine the mass of the compact object and its distance from Earth. The UC Berkeley-led team estimated that it is between 2,280 and 6,260 light-years (700-1920 parsecs) away, toward the center of the Milky Way galaxy and near the large bulge that surrounds the galaxy’s central massive black holes.

The STScI group estimated that it is approximately 5,153 light-years (1,580 parsecs) away.

Looking for a needle in a haystack

Lu and Lam first became interested in the object in 2020 after the STScI team tentatively concluded that five microlensing events observed by Hubble – which all lasted for more than 100 days and thus could have been black holes – may not be caused by compact objects after all.

Lu, who has been looking for free-flowing black holes since 2008, believed the data would help her better estimate their abundance in the galaxy, which is roughly estimated at between 10 million and 1 billion. To date, star-sized black holes have only been found as part of binary star systems. Black holes in binary are seen either in X-rays, which are produced when material from the star falls on the black hole, or by new gravitational wave detectors, which are sensitive to merging of two or more black holes. But these events are rare.

“Casey and I saw data and we got really interested. We said, ‘Wow, no black holes.’ It’s amazing, even if it should have been, “Lu said.” And so we started looking at data. If there really were no black holes in the data, then this would not match our model for how many black holes there should be in the Milky Way. Something would need to change in our understanding of black holes – either their numbers or how fast they move or their masses. “

When Lam analyzed the photometry and astrometry of the five microlensing events, she was surprised that one, OB110462, had the properties of a compact object: The lens object appeared dark, and therefore not a star; the star’s brightness lasted a long time, almost 300 days; and the distortion of the background star’s position was also prolonged.

The length of the lensing event was the most important tip, Lam said. In 2020, she showed that the best way to search for black hole microlenses was to look for very long events. Only 1% of detectable microlensing events are likely to come from black holes, she said, so looking at all the events would be like searching for a needle in a haystack. But, Lam estimated, about 40% of microlensing events lasting more than 120 days are likely black holes.

“How long the brighter event lasts is a hint of how massive the foreground lens that bends the light from the background star is,” Lam said. “Long events are more likely due to black holes. However, there is no guarantee, as the duration of the brightening episode depends not only on how massive the foreground lens is, but also on how fast the foreground lens and background star move relative. apparent position, we can confirm if the foreground lens is really a black hole. “

According to Lu, the gravitational effect of OB110462 on the light from the background star was incredibly long. It took about a year for the star to brighten to its peak in 2011, then about a year to dampen back to normal.

More data will separate black holes from neutron stars

To confirm that OB110462 was caused by a super-compact object, Lu and Lam asked for more astrometric data from Hubble, some of which arrived in October last year. The new data showed that the change in the star’s position as a result of the lens’ gravitational field is still observable 10 years after the event. Additional Hubble observations of the microlens are tentatively planned for the fall of 2022.

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

Lu and Lam suspect that the two teams’ different conclusions are due to the fact that the astrometric and photometric data provide different measures of the relative movements of the foreground and background objects. The astrometric analysis also differs between the two teams. The UC Berkeley-led team claims that it is not yet possible to distinguish whether the object is a black hole or a neutron star, but they hope to solve the discrepancy with more Hubble data and improved analysis in the future.

“As much as we would like to say that it is definitely a black hole, we must report all permissible solutions. This includes both lower holes with lower mass and possibly even a neutron star,” Lu said.

“If you can not believe the light curve, the brightness, then it says something important. If you do not believe in the position versus 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. Photometry and astrometry data are derived from the same physical process, which means the brightness and the position must be consistent with each other. So there is something missing there. “

Both teams also appreciated the speed of the super-compact lens object. The Lu / Lam team found a relatively calm speed, less than 30 kilometers per second. The STScI team found an unusually high speed, 45 km / s, which it interpreted as a result of an extra kick that the alleged black hole received from the supernova that generated it.

Lu interprets her team’s low speed estimate as potential 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 do not give 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 FINESST 80NSSC21K2043).


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