June 19, 2024

Stellar “Ghost” Discovered: Astronomers May Have Detected a “Dark” Free-Floating Black Hole

If, as astronomers think, the death of big stars leaves behind great voids, there ought to be numerous millions of them scattered throughout the Milky Way galaxy. The problem is, separated great voids are invisible.
Now, a team led by University of California, Berkeley, astronomers has for the very first time discovered what might be a free-floating black hole by observing the lightening up of a more distant star as its light was distorted by the objects strong gravitational field– so-called gravitational microlensing.
The group, led by graduate 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. Since astronomers believe that the leftover remnant of a dead star need to be heavier than 2.2 solar masses in order to collapse to a black hole, the UC Berkeley scientists caution that the things might be a neutron star instead of a black hole. Neutron stars are also thick, extremely compact items, however their gravity is stabilized by internal neutron pressure, which prevents more collapse to a great void.
Whether a black hole or a neutron star, the things is the very first dark outstanding residue– an outstanding “ghost”– found wandering through the galaxy unpaired with another star.
Hubble Space Telescope image of a remote star that was brightened and misshaped by an undetectable however heavy and really compact object between it and Earth. The compact things– estimated by UC Berkeley astronomers to be between 1.6 and 4.4 times the mass of our sun– might be a free-floating great void, among possibly 200 million in the Milky Way galaxy. Credit: Image thanks to STScI/NASA/ESA
” This is the first free-floating black hole or neutron star discovered with gravitational microlensing,” Lu stated. “With microlensing, were able to penetrate these lonely, compact items and weigh them. I think we have actually opened a brand-new window onto these dark objects, which cant be seen any other method.”
Identifying how many of these compact things occupy the Milky Way galaxy will assist astronomers comprehend the advancement of stars– in particular, how they die– and of our galaxy, and possibly reveal whether any of the hidden black holes are primordial great voids, which some cosmologists think were produced in big quantities throughout the Big Bang.
The analysis by Lam, Lu and their global group has been accepted for publication in The Astrophysical Journal Letters. The analysis consists of four other microlensing occasions that the group concluded were not brought on by a black hole, though two were most likely triggered by a white dwarf or a neutron star. The group also concluded that the likely population of great voids in the galaxy is 200 million– about what most theorists anticipated.
Same data, different conclusions
Especially, a completing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing occasion and declares that the mass of the compact item is better to 7.1 solar masses and indisputably a great void. A paper explaining the analysis by the STScI group, led by Kailash Sahu, has actually been accepted for publication in The Astrophysical Journal.
Both groups utilized the very same information: photometric measurements of the far-off stars brightening as its light was distorted or “lensed” by the super-compact things, and astrometric measurements of the moving of the distant stars place in the sky as an outcome of the gravitational distortion by the lensing things. The photometric data came from two microlensing surveys: the Optical Gravitational Lensing Experiment (OGLE), which employs a 1.3-meter telescope in Chile run by Warsaw University, and the Microlensing Observations in Astrophysics (MOA) experiment, which is installed on a 1.8-meter telescope in New Zealand run by Osaka University. The astrometric information originated from NASAs Hubble Space Telescope. STScI handles the science program for the telescope and performs its science operations.
Because both microlensing studies captured the same item, it has 2 names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, for short.
While studies like these find about 2,000 stars lightened up by microlensing each year in the Milky Way galaxy, the addition of astrometric data is what allowed the 2 groups to figure out the mass of the compact things and its distance from Earth. The UC Berkeley-led group approximated that it lies between 2,280 and 6,260 light years (700-1920 parsecs) away, in the instructions of the center of the Milky Way Galaxy and near the large bulge that surrounds the galaxys main huge great void.
The STScI group approximated that it lies about 5,153 light years (1,580 parsecs) away.
Trying to find a needle in a haystack
Lu and Lam first became thinking about the object in 2020 after the STScI group tentatively concluded that 5 microlensing occasions observed by Hubble– all of which lasted for more than 100 days, and thus could have been great voids– may not be triggered by compact objects after all.
To date, star-sized black holes have been discovered only as part of binary star systems. Black holes in binaries are seen either in X-rays, produced when product from the star falls onto the black hole, or by recent gravitational wave detectors, which are sensitive to mergers of two or more black holes.
We stated, Wow, no black holes. If there were actually no black holes in the information, then this would not match our model for how many black holes there ought to be in the Milky Way. Something would have to alter in our understanding of black holes– either their number or how quick they move or their masses.”
When Lam evaluated the photometry and astrometry for the 5 microlensing occasions, she was surprised that one, OB110462, had the attributes of a compact things: The lensing object seemed dark, and thus not a star; the excellent lightening up lasted a long period of time, almost 300 days; and the distortion of the background stars position also was long-lasting.
The length of the lensing event was the main tipoff, Lam said. In 2020, she showed that the very best way to search for black hole microlenses was to try to find long events. Just 1% of detectable microlensing occasions are likely to be from black holes, she said, so taking a look at all occasions would resemble looking for a needle in a haystack. However, Lam calculated, about 40% of microlensing occasions that last more than 120 days are likely to be black holes.
” How long the lightening up event lasts is a hint of how massive the foreground lens flexing the light of the background star is,” Lam said. “Long occasions are most likely due to black holes. Its not a warranty, though, due to the fact that the period of the brightening episode not only depends upon how enormous the foreground lens is, but likewise on how quick the foreground lens and background star are moving relative to each other. By also getting measurements of the apparent position of the background star, we can verify whether the foreground lens truly is a black hole.”
According to Lu, the gravitational impact of OB110462 on the light of the background star was remarkably long. It took about one year for the star to lighten up to its peak in 2011, then about a year to dim back to normal.
More data will differentiate black hole from neutron star
To confirm that OB110462 was triggered by a super-compact item, Lu and Lam asked for more astrometric information from Hubble, some of which got here last October. That new data showed that the modification in position of the star as a result of the gravitational field of the lens is still observable 10 years after the occasion. Further Hubble observations of the microlens are tentatively arranged for fall 2022.
Analysis of the new information confirmed that OB110462 was likely a black hole or neutron star.
Lu and Lam think that the differing conclusions of the two teams are due to the truth that the astrometric and photometric data offer various measures of the relative motions of the foreground and background items. The astrometric analysis likewise varies between the 2 teams. The UC Berkeley-led group argues that it is not yet possible to differentiate whether the object is a black hole or a neutron star, but they want to deal with the inconsistency with more Hubble data and enhanced analysis in the future.
” As much as we want to say it is definitively a black hole, we should report all permitted solutions. This includes both lower mass black holes and possibly even a neutron star,” Lu stated.
” If you cant think the light curve, the brightness, then that states something essential. If you dont believe the position versus time, that informs you something important,” Lam stated. “So, if one of them is incorrect, we have to understand why. Or the other possibility is that what we determine in both data sets is correct, however our design is incorrect. The photometry and astrometry data arise from the very same physical process, which means the brightness and position should follow each other. Theres something missing there. ”
Both groups likewise estimated the velocity of the super-compact lensing item. The Lu/Lam team discovered a reasonably sedate speed, less than 30 kilometers per second. The STScI team found an abnormally large velocity, 45 km/s, which it interpreted as the outcome of an additional kick that the supposed black hole received from the supernova that generated it.
Lu interprets her groups low speed quote as potentially supporting a new theory that great voids are not the result of supernovas– the reigning assumption today– however instead originated from stopped working supernovas that do not make an intense splash in deep space or offer the resulting black hole a kick.
Reference: “An isolated mass gap great void or neutron star detected with astrometric microlensing” by Casey Y. Lam, Jessica R. Lu, Andrzej Udalski, Ian Bond, David P. Bennett, Jan Skowron, Przemek Mroz, Radek Poleski, Takahiro Sumi, Michal K. Szymanski, Szymon Kozlowski, Pawel Pietrukowicz, Igor Soszynski, Krzysztof Ulaczyk, Lukasz Wyrzykowski, Shota Miyazaki, Daisuke Suzuki, Naoki Koshimoto, Nicholas J. Rattenbury, Matthew W. Hosek Jr., Fumio Abe, Richard Barry, Aparna Bhattacharya, Akihiko Fukui, Hirosane Fujii, Yuki Hirao, Yoshitaka Itow, Rintaro Kirikawa, Iona Kondo, Yutaka Matsubara, Sho Matsumoto, Yasushi Muraki, Greg Olmschenk, Clement Ranc, Arisa Okamura, Yuki Satoh, Stela Ishitani Silva, Taiga Toda, Paul J. Tristram, Aikaterini Vandorou, Hibiki Yama, Natasha S. Abrams, Shrihan Agarwal, Sam Rose and Sean K. Terry, Accepted, The Astrophysical Journal Letters.arXiv:2202.01903.
The work of Lu and Lam is supported by the National Science Foundation (1909641) and the National Aeronautics and Space Administration (NNG16PJ26C, NASA FINESST 80NSSC21K2043).

Because astronomers think that the remaining residue of a dead star must be heavier than 2.2 solar masses in order to collapse to a black hole, the UC Berkeley scientists caution that the item might be a neutron star instead of a black hole. Neutron stars are also dense, extremely compact items, however their gravity is balanced by internal neutron pressure, which avoids more collapse to a black hole.
Black holes in binaries are seen either in X-rays, produced when product from the star falls onto the black hole, or by recent gravitational wave detectors, which are delicate to mergers of 2 or more black holes. If there were actually no black holes in the information, then this would not match our design for how lots of black holes there should be in the Milky Way. The UC Berkeley-led group argues that it is not yet possible to identify whether the things is a black hole or a neutron star, however they hope to resolve the inconsistency with more Hubble data and enhanced analysis in the future.

Artists illustration of a great void.
Gravitational microlensing shows up great void candidate, one of 200 million in the galaxy.
When huge stars come to the end of their lives and take off in a supernova, they leave a great void. It is estimated that about one in a thousand stars is huge enough to offer birth to a black hole. With the Milky Way being home to an approximated 100 to 400 billion stars, there are likely a vast number of great voids throughout our galaxy.
Yet black holes by their very nature can be very difficult to discover, especially if they are separated. A black hole has such effective gravity that light doesnt leave, so we usually detect them by their gravitational influence on other objects or by radiation created by the surrounding matter they are feasting on. Without close-by things or accreting matter, there could be numerous millions of great voids throughout our galaxy that are essentially undetectable to astronomers.