Rare Discovery: Astronomers Think They’ve Spotted First Free-Floating Black Hole

If big stars die and leave behind black holes, as astronomers believe, there should be hundreds of millions of them distributed throughout the Milky Way galaxy. The issue is that isolated black holes are undetectable.

Now, astronomers from the University of California, Berkeley, have discovered what may be a free-floating black hole for the first time by viewing the illumination of a more distant galaxy as its light was disrupted by the object’s strong gravitational field—a process known as gravitational microlensing.

The team, led by UC Berkeley assistant professor of astronomy Jessica Lu and graduate student Casey Lam, calculates that the mass of the undetectable compact object is between 1.6 and 4.4 times that of the sun.

The UC Berkeley researchers warn that the object could be a neutron star rather than a black hole because astronomers believe the relic of a dead star must be heavier than 2.2 solar masses in order to collapse to a black hole.

Neutron stars are also tight, compact objects, but their gravity is counterbalanced by internal neutron pressure, which prevents them from collapsing further into black holes.

Whether it’s a black hole or a neutron star, the item is the galaxy’s first dark stellar remnant, a spectacular “ghost” discovered roaming around alone.

“With gravitational microlensing, this is the first free-floating black hole or neutron star identified,” Lu said. “We can explore and weigh these lonely, small objects using microlensing. I believe we have opened a new window into these dark objects that were previously unseen.”

Read: Unusual Radio Structures Discovered Around The Brightest Quasar Ever Found (3C 273)

How Will These Help Researchers?

Determining the number of these compact objects in the Milky Way galaxy will aid astronomers in better understanding the evolution of stars—particularly how they die—as well as our galaxy’s evolution.

It may also reveal whether any of the unseen black holes are primordial black holes, which some cosmologists believe were produced in large quantities during the Big Bang.

The Astrophysical Journal Letters has accepted Lam, Lu, and their multinational team’s analysis for publication. The scientists concluded that four more microlensing events were not created by a black hole, albeit two of them were most likely caused by a white dwarf or neutron star.

The scientists also determined that the galaxy’s expected population of black holes is 200 million, which is similar to what most theorists projected.

Different Conclusions Based On The Same Facts

A competing study from Baltimore’s Space Telescope Science Institute (STScI) investigated the identical microlensing event and argues that the compact object’s mass is closer to 7.1 solar masses, indicating that it is unmistakably a black hole. The Astrophysical Journal has approved an article by the STScI team, lead by Kailash Sahu, that describes the analysis.

Both teams were using the same data: photometric measurements of the distant star’s brightness as its light was deformed or “lensed” by the super-compact objects, and astrometric measurements of the distant star’s position in the sky shifting as a result of the lensing object’s gravitational distortion.

Related: Discover The Undiscover: Webb’s Quest For Primeval Black Holes

The Photometric Data

The photometric data came from two microlensing surveys: the Optical Gravitational Lensing Experiment (OGLE), which is run by Warsaw University and uses a 1.3-meter telescope in Chile, and the Microlensing Observations in Astrophysics (MOA), which is run by Osaka University and uses a 1.8-meter telescope in New Zealand.

NASA’s Hubble Space Telescope provided the astrometric data. STScI is in charge of the telescope’s science programme and science operations.

Images Captured By The Microlensing Surveys

MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, are the two names for the same object captured by both microlensing surveys.

While surveys like this find roughly 2,000 stars in the Milky Way galaxy that have been intensified by microlensing each year, it was the addition of astrometric data that allowed the two teams to establish the compact object’s mass and distance from Earth.

It is believed to be between 2,280 and 6,260 light years (700-1920 parsecs) away, in the direction of the Milky Way’s core and near the huge bulge that surrounds the galaxy’s central big black hole, according to the UC Berkeley-led research. It is estimated to be 5,153 light years (1,580 parsecs) distant by the STScI group.

In Search Of A Needle In A Haystack

After the STScI team initially decided that five microlensing events seen by Hubble—all of which lasted more than 100 days and thus could have been black holes—might not be generated by compact objects after all, Lu and Lam got interested in the object in 2020.

Lu, who has been seeking free-floating black holes since 2008, hoped the data would help her better estimate their number in the galaxy, which is estimated to be between 10 million and 1 billion.

Star-sized black holes have only been discovered as part of binary star systems so far. X-rays, which are created as material from the star falls onto the black hole, or modern gravitational wave detectors, which are sensitive to mergers of two or more black holes, can be spotted in binaries. However, such occurrences are uncommon.

“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 were really no black holes in the data, then this wouldn’t match our model for 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.”

What Was The Interesting Discovery?

When Lam looked at the photometry and astrometry for the five microlensing events, she discovered that one of them, OB110462, showed compact object characteristics: The lensing object appeared black and thus not to be a star; the stellar brightening lasted about 300 days; and the distortion of the background star’s position persisted for the same amount of time.

What Was The Main Indicator?

The main indicator, according to Lam, was the length of the filming event. She demonstrated in 2020 that looking for very long events was the best technique to find black hole microlenses.

According to her, black holes account for only 1% of measurable microlensing events, so looking at all of them would be like looking for a needle in a haystack.

However, according to Lam, around 40% of microlensing occurrences lasting more than 120 days are likely to be black holes.

“How long the brightening event lasts is a hint of how massive the foreground lens bending the light of the background star is,” Lam said. “Long events are more likely due to black holes.

It’s not a guarantee, though, because the duration of the brightening episode not only depends on how massive the foreground lens is, but also on how fast the foreground lens and background star are moving relative to each other.

However, by also getting measurements of the apparent position of the background star, we can confirm whether the foreground lens really is a black hole.”

According to Lu, OB110462’s gravitational influence on the light of the background star lasted an incredibly long time. The star brightened for about a year until it reached its peak in 2011, then dimmed for about a year before returning to normal.

More Information Will Be Needed To Distinguish Between A Black Hole And A Neutron Star.

Lu and Lam requested more astrometric data from Hubble to corroborate that OB110462 was caused by a super-compact object, and some of it arrived in October.

The new data revealed that the shift in the star’s position caused by the lens’ gravitational field may still be seen 10 years after the occurrence. The Hubble Space Telescope will observe the microlens again in the fall of 2022.

According to the new evidence, OB110462 is most likely a black hole or neutron star.

Why Is Astrometric And Photometric Data Different?

The fact that the astrometric and photometric data yield different measurements of the relative movements of the foreground and background objects, Lu and Lam believe, is to blame for the two teams’ divergent conclusions. The astrological analysis of the two teams also differs.

The UC Berkeley-led team claims that it is currently impossible to tell whether the object is a black hole or a neutron star, but that more Hubble data and enhanced analysis will help them settle the discrepancy in the future.

“We must disclose all possible solutions, as much as we would prefer to say it is clearly a black hole. This might encompass both low-mass black holes and even neutron stars “Lu expressed his thoughts.

“If you can’t believe the light curve, the brightness, then that says something important. If you don’t believe the position versus time, that tells you something important,” Lam said. “So, if one of them is wrong, we have to understand why. Or the other possibility is that what we measure in both data sets is correct, but our model is incorrect. The photometry and astrometry data arise from the same physical process, which means the brightness and position must be consistent with each other. So, there’s something missing there.”


The velocity of the super-compact lensing object was likewise calculated by both teams. The Lu/Lam team discovered a rather slow speed of less than 30 km/s. The STScI team discovered an exceptionally high velocity of 45 km/s, which they interpreted as the result of the alleged black hole receiving an extra kick from the supernova that created it.

Lu sees her team’s low velocity estimate as confirming a novel notion that black holes are formed by unsuccessful supernovas that don’t make a bright splash in the universe or give the following black hole a kick, rather than supernovas, which are the current assumption.


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