Astronomers may have discovered a ‘dark’ free-floating black hole

image: Hubble Space Telescope image of a distant star that was brightened and distorted by an invisible but very compact and massive object between the star and Earth. The compact object — estimated by UC Berkeley astronomers to be 1.6 to 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.
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Credit: Image Courtesy of STScI/NASA/ESA

If, as astronomers believe, the death of large stars leaves black holes behind, then hundreds of millions of them should be scattered all over the Milky Way galaxy. 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 could be a free-floating black hole by observing the brightening of a more distant star as its light was distorted by the object’s strong gravitational field. – so – so-called. gravity microlensing.

The team, led by graduate student Casey Lam and Jessica Lu, an associate professor of astronomy at UC 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 that the leftover remnant of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, the UC Berkeley researchers warn that the object could be a neutron star rather than a black hole. Neutron stars are also dense, very compact objects, but their gravity is offset 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 the galaxy without mating with another star.

“This is the first free-floating black hole or neutron star discovered with gravitational microlensing,” Lu said. “With microlens, we can examine and weigh these solitary, compact objects. I think we’ve opened a new window on these dark objects, which can’t be seen any other way.”

By determining how many of these compact objects populate the Milky Way Galaxy, astronomers can understand the evolution of stars — particularly how they die — and our galaxy, and perhaps reveal whether any of the invisible black holes are primordial black holes, which some cosmologists believe. that they were produced in large quantities during the Big Bang.

The analysis of Lam, Lu and their international team has been accepted 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 likely caused by a white dwarf or a neutron star. The team also concluded that the likely population of black holes in the galaxy is 200 million — roughly what most theorists predicted.

Same data, different conclusions

Notably, a competing team at the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event, claiming that the compact object’s mass is closer to 7.1 solar masses and is indisputably 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 brightening of the distant star as the light was distorted or “lensed” by the supercompact object, and astrometric measurements of the shift of the distant star’s location in the sky due to gravity. distortion by the lens object. The photometric data comes from two microlensing 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. meter telescope in New Zealand, operated by the University of Osaka. The astrometric data comes from NASA’s Hubble Space Telescope. STScI manages the scientific program for the telescope and carries out the scientific activities.

Since both microlensing studies 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 discover about 2,000 stars that are getting brighter each year by microlensing in the Milky Way Galaxy, the addition of astrometric data is what allowed the two teams to determine the compact object’s mass and its distance from Earth. The UC Berkeley-led team estimated it to be located between 2,280 and 6,260 light-years (700-1920 parsecs) toward the center of the Milky Way and near the large bulge that forms the galaxy’s central massive black hole. surrounds.

The STScI group estimated it to be about 5,153 light-years (1580 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 – all of which lasted more than 100 days and thus could be black holes – may not be caused by compact objects finally.

Lu, who has been searching for free-floating black holes since 2008, thought the data would help her better estimate their abundance in the galaxy, which is roughly estimated to be 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 stars are seen either by X-rays, produced when material from the star falls onto the black hole, or by recent gravitational wave detectors, which are sensitive to the fusion of two or more black holes. But these events are rare.

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

When Lam analyzed the photometry and astrometry for the five microlensing events, she was surprised that one, OB110462, had the characteristics of a compact object: the lens object appeared dark and thus not a star; the stellar clearing took a long time, nearly 300 days; and the distortion of the background star’s position was also prolonged.

The length of the lens 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 likely come from black holes, she said, so looking at all the events would be like looking for a needle in a haystack. But, Lam calculated, about 40% of microlensing events lasting longer than 120 days are likely black holes.

“How long the brightening event lasts is a hint of how massive the foreground lens is that bends the light from the background star,” Lam said. “Long events are more likely due to black holes. However, it is not a guarantee, because the duration of the brightening episode depends not only on how heavy the foreground lens is, but also on how fast the foreground lens and the background star move relatively. By measuring the position of the background star, we can confirm whether the foreground lens is really a black hole.”

According to Lu, OB110462’s gravitational influence on the background star’s light was astonishingly long. It took about a year for the star to brighten to its peak in 2011, and then about a year to dim back to normal.

More data will distinguish black hole from neutron star

To confirm that OB110462 was caused by a super-compact object, Lu and Lam requested more astrometric data from Hubble, some of which arrived last October. Those new data showed that the change in the star’s position due to the lens’ gravitational field is still discernible 10 years after the event. Further Hubble observations of the microlens are tentatively scheduled for fall 2022.

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

Lu and Lam suspect that the two teams’ different conclusions are due to the fact that the astrometric and photometric data give different measures of the relative motions of the foreground and background objects. The astrometric analysis also differs between the two teams. The UC Berkeley-led team says it’s 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’d like to say it’s definitely a black hole, we need to report all allowed solutions. This includes both lower-mass black holes and possibly even a neutron star,” Lu said.

“If you can’t believe the light curve and the brightness, that says something important. If you don’t believe the position versus time, that says 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 datasets is correct, but our model is incorrect. The photometry and astrometry data comes from the same physical process, meaning that the brightness and position should be consistent with each other. So something is missing there.”

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

Lu interprets her team’s low-velocity estimate as possible support for a new theory that black holes aren’t the result of supernovas — the current prevailing assumption — but instead come from failed supernovas that don’t make a bright splash in the universe or resulting Black give a kick hit.

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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