By definition, black holes are invisible unless they are part of a star binary or are surrounded by an accretion disk. Although most stellar-sized black holes aren’t, astronomers have been looking for them through gravitational microlensing events, in which the black hole intensifies and bends light from nearby stars as it travels toward the galactic center. A team led by UC Berkeley may have discovered the first free-floating black hole, while further evidence is needed to rule out the possibility of a neutron star.
If big stars die and leave behind black holes, as scientists 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, a team led by astronomers from the University of California, Berkeley, has discovered what appears to 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 — a process known as gravitational microlensing.
The team, lead by UC Berkeley assistant professor of astronomy Jessica Lu and graduate student Casey Lam, calculates that the mass of the unseen compact object is between 1.6 and 4.4 times that of the sun. The UC Berkeley researchers warn that the object might be a neutron star rather than a black hole since scientists 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 likewise tight, compact objects, but their gravity is counterbalanced by internal neutron pressure, which prevents them from collapsing further into black holes.
The item is the first dark stellar remnant — a stellar “ghost” — observed roaming around the galaxy unpaired with another star, whether it be a black hole or a neutron star.
“With gravitational microlensing, this is the first free-floating black hole or neutron star identified,” Lu added. “We can explore and weigh these lonely, small objects using microlensing. I believe we have created a new window into these dark things that were previously unseen.”
Determining the number of these compact objects in the Milky Way galaxy will aid astronomers in better understanding the evolution of stars and our galaxy, as well as reveal whether any of the unseen black holes are primordial black holes, which some cosmologists believe were produced in large numbers during the Big Bang.The Astrophysical Journal Letters has approved Lam, Lu, and their multinational team’s study for publication.
The scientists determined 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 number of black holes is 200 million, which is similar to what most theorists projected.
Different conclusions based on the same facts
A rival 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 study.Both teams utilized the same data: photometric measurements of the distant star’s brightness as its light was distorted or “lensed” by the super-compact object, and astrometric measurements of the distant star’s position in the sky altering as a result of the lensing object’s gravitational distortion. 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 program and science operations.
MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, are the two designations 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 inclusion 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) distant, 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
Lu and Lam got interested in the item in 2020 when the STScI team provisionally decided that five Hubble microlensing events — all of which lasted more than 100 days and so may have been black holes — were not, after all, generated by compact objects.Lu, who has been searching for free-floating black holes since 2008, hoped the data would help her better estimate their number in the galaxy, which is believed 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.
“When Casey and I first saw the statistics, we were enthralled. ‘Wow, no black holes,’ we said. Even if there should have been,’ that’s incredible “Lu expressed his thoughts. “As a result, we began to examine the data. If there were no black holes in the data, our model for how many black holes there should be in the Milky Way would be off. Something about how we think about black holes would have to alter, whether it’s the quantity of them, how fast they travel, or how big they are.”
When Lam looked at the photometry and astrometry for the five microlensing events, she was shocked to find that one of them, OB110462, showed compact object characteristics: The lensing object seemed black and therefore not to be a star; the stellar brightening lasted about 300 days; and the distortion of the background star’s location persisted for the same amount of time.
The length of the filming session was the primary indicator, according to Lam. 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 just 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 greater than 120 days are likely to be black holes.
“How long the brightening event lasts gives us an idea of how large the foreground lens is that bends the light of the background star,” Lam explained. “Black holes are more likely to cause long events. It’s not a certainty, though, because the length of the brightening episode is determined not only by the mass of the front lens, but also by how quickly the foreground lens and background star move relative to one another. However, we can establish if the foreground lens is a black hole by taking measurements of the apparent location of the background star.”
According to Lu, OB110462’s gravitational impact on the light of the background star lasted an incredibly long time. The star brightened for approximately 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 further astrometric data from Hubble to corroborate that OB110462 was produced by a super-compact object, and part of it came in October. The new data revealed that the shift in the star’s location 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.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 results. The astrological analysis of the two teams also differs. The UC Berkeley-led team claims that it is now 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 disagreement in the future.
“We must disclose all possible answers, as much as we would prefer to claim 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 brightness or the light curve, then something is wrong. If you don’t trust the position versus time, you’re missing out on something crucial “Lam expressed his thoughts. “So, if one of them is incorrect, we must figure out why. Another alternative is that we are measuring the same thing in both data sets, but our model is inaccurate. Because photometry and astrometry data are generated by the same physical mechanism, the brightness and location must be constant. As a result, something is lacking.”
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 consequence of the alleged black hole receiving an additional 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 failed supernovas that don’t produce a brilliant splash in the cosmos or give the subsequent black hole a kick, rather than supernovas, which are the current assumption.