Black holes are, by their nature, invisible unless they are part of a stellar binary or surrounded by an accretion disk.
Most stellar-sized black holes are not, but astronomers have been searching for them through microgravity events, where the black hole brightens and distorts light from stars toward the galactic center.
A team led by the University of California at Berkeley may have discovered the first floating black hole, although more data is needed to rule out the presence of a neutron star.
If, as astronomers believe, the death of large stars leaves behind black holes, there should be hundreds of millions of them scattered throughout the Milky Way. The problem is that isolated black holes are not visible.
Now, a team led by the University of California, Berkeley, astronomers have discovered for the first time what could be a floating black hole by observing the brightness of a farther star as its light is distorted by an object's strong gravitational field — the so-called microgravity.
The team, led by UC Berkeley astronomy professor Jessica Lu and graduate student Casey Lamm, 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 remnants 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 very dense and compact objects as well, but their gravity is balanced by internal neutron pressure, preventing them from collapsing further 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 through the galaxy unassociated with another star.
"This is the first time that a floating black hole or neutron star has been detected using microgravitational lenses," Lu said. "We can examine and weigh these isolated, compressed objects with the finer lens. I think we've opened a new window on these dark objects, which cannot be seen any other way."
Determining how many of these compact objects inhabit the Milky Way will help astronomers understand the evolution of stars — in particular, how they die — and our galaxy, and perhaps reveal whether any of the unseen black holes are primordial black holes, which some believe. Cosmologists believe that it was produced in large quantities during the Big Bang.
Lamm, Lu, and their international team's research have 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 are likely caused by a white dwarf or a neutron star.
The team also determined that the likely number of black holes in the galaxy is 200 million, which is close to what most theorists predicted.
Notably, a competing team from the Space Telescope Science Institute (STScI) in Baltimore examined the same microlensing event and claimed that the compact object's mass is closer to 7.1 solar masses and that it is an undeniable black hole.
The Astrophysical Journal has accepted a paper describing the analysis conducted by the STScI team led by Kailash Sahu.
Both teams used the same data: photometric measurements of the brightness of a distant star as its light was distorted or "reflected" by the highly compressed object, and astronomical measurements of the distant star's changing position in the sky due to gravity.
The optical data came from two microlensing surveys: the Optical Gravitational Lens Experiment (OGLE) in Chile, operated by the University of Warsaw, and the Microlens Observations in Astrophysics (MOA) in New Zealand, also operated by the University of Warsaw.
University of Osaka NASA's Hubble Space Telescope provided the astronomical data. STScI oversees the telescope's science program and science operations.
Because both precision lens reconnaissance captured the same object, it is known by two different names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462.
While surveys like this one discover about 2,000 bright stars by microlensing each year in the Milky Way, The inclusion of astronomical data enabled the two teams to calculate the compact object's mass and distance from Earth.
It is estimated to be between 2,280 and 6260 light-years away (700-1920 parsecs) from the center of the Milky Way, near the large bulge that surrounds the galaxy's central supermassive black hole.
The STScI cluster has been estimated to be about 5,153 light-years (1,580 parsecs) away.
Lu and Lam first became interested in the object in 2020 after the STScI team initially concluded that five microlensing events observed by Hubble - all of which lasted for more than 100 days and thus could be black holes - may not have been caused by compact objects yet. everything.
Lu, who has been searching for free-moving black holes since 2008, thought the data would help her better estimate their abundance in the galaxy, which was roughly estimated to be between 10 million and 1 billion.
Only star-sized black holes have been discovered as part of binary star systems so far. Binaries of black holes can be seen in X-rays, which are produced when material from a star falls onto a black hole, or by modern gravitational wave detectors, which are vulnerable to the merger of two or more black holes These occurrences, however, are uncommon.
data. If there were no black holes in the data, our model of how many black holes should be present in the Milky Way would be incorrect. Something will have to change in the understanding of black holes - either their number, how fast they are, or how massive they are. "
When Lahm examined the photometric and astrometry of the five-minute lens events, I was surprised to discover that one, OB110462, exhibited the characteristics of a compact body: the lens body appeared dark., and therefore not a star; stellar brightness lasted for a long time, almost 300 days; The distortion of the background star's position was also long-term.
The main tip, according to Lamm, was the length of the lens event. It was discovered in 2020 that the best way to look for black hole microlenses is to look for very long events.
According to her, only 1% of the minute lens events that can be detected are likely to be from black holes, so looking at all of them would be like looking for a needle in a haystack. However, according to Lamm, approximately 40% of microlensing events that last more than 120 days are likely to be black holes.
"How long the bright event lasts is an indication of how massive the foreground lens bends the light of the background star," Lamm explained. "Longer events are almost certainly caused by black holes.
" This is not a guarantee, because the duration of the bright ring is determined not only by the size of the foreground lens but also by the rate at which the foreground lens and background star move relative to each other.
We can confirm whether the foreground lens is a black hole by also obtaining measurements for the apparent location of the background star."
According to Lu, the gravitational effect of OB110462 on the background star's light was surprisingly long. It took about a year for the star to reach its peak in 2011, and another year to return to normal.
More data will distinguish a black hole from a neutron star
Low and Lam requested more astronomical data from Hubble to confirm that OB110462 was caused by an extremely compact object, and some of it arrived last October.
This new data showed that the change in the star's position caused by the gravitational field of the lens could still be observed ten years later. More Hubble observations of microlensing are planned for the fall of 2022.
The new information confirmed that OB110462 was most likely a black hole or neutron star.
Low and Lam suspect that the disparity in conclusions between the two teams is due to the fact that the astronomical and photometric data provide different measures of the relative motions of the fore and aft objects. The astrological analysis also differs between the two teams.
The UC Berkeley team claims that it is currently impossible to tell whether the object is a black hole or a neutron star, but they hope to resolve the discrepancy in the future with more Hubble data and improved analysis.
"As much as we would definitively say it's a black hole," Lu said, "we should report all permissible solutions." "This includes both low-mass black holes and possibly even neutron stars."
"If you can't believe the light's curve or brightness, it means something important. If you can't believe the situation versus time, it means something is wrong "Lamm stated. "So, if one of them is incorrect, we must figure out why.
" Another possibility is that the measurements in the two data sets are accurate, but our model is incorrect. Because photometric and astrometric data are derived from the same physical process, brightness and position must be consistent. With one another. So something is missing there."
The velocity of the ultrafine lens body was also estimated by both groups. The Lu/Lam team discovered a relatively slow speed of fewer than 30 kilometers per second. The STScI team discovered an unusually high speed, 45 km/s, which they interpreted as the result of an extra kick from the supernova it produced.
LHer team's low-velocity estimate is interpreted as possible 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 don't make a bright burst in the universe or give the resulting black hole a kick.
Source:Materials provided by University of California - Berkeley.References: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, Sean K. Terry. An isolated mass gap black hole or neutron star detected with astrometric microlensing. Accepted to APJ Letters, 2022 [abstract]Kailash C. Sahu, Jay Anderson, Stefano Casertano, Howard E. Bond, Andrzej Udalski, Martin Dominik, Annalisa Calamida, Andrea Bellini, Thomas M. Brown, Marina Rejkuba, Varun Bajaj, Noe Kains, Henry C. Ferguson, Chris L. Fryer, Philip Yock, Przemek Mroz, Szymon Kozlowski, Pawel Pietrukowicz, Radek Poleski, Jan Skowron, Igor Soszynski, Michael K. Szymanski, Krzysztof Ulaczyk, Lukasz Wyrzykowski, Richard Barry, David P. Bennett, Ian A. Bond, Yuki Hirao, Stela Ishitani Silva, Iona Kondo, Naoki Koshimoto, Clement Ranc, Nicholas J. Rattenbury, Takahiro Sumi, Daisuke Suzuki, Paul J. Tristram, Aikaterini Vandorou, Jean-Philippe Beaulieu, Jean-Baptiste Marquette, Andrew Cole, Pascal Fouque, Kym Hill, Stefan Dieters, Christian Coutures, Dijana Dominis-Prester, Clara Bennett, Etienne Bachelet, John Menzies, Michael Alb-row, Karen Pollard, Andrew Gould, Jennifer Yee, William Allen, Leonardo Andrade de Almeida, Grant Christie, John Drummond, Avishay Gal-Yam, Evgeny Gorbikov, Francisco Jablonski, Chung-Uk Lee, Dan Maoz, Ilan Manulis, Jennie McCormick, Tim Natusch, Richard W. Pogge, Yossi Shvartzvald, Uffe G. Jorgensen, Khalid A. Alsubai, Michael I. Andersen, Valerio Bozza, Sebastiano Calchi Novati, Martin Burgdorf, Tobias C. Hinse, Markus Hundertmark, Tim-Oliver Husser, Eamonn Kerins, Penelope Longa-Pena, Luigi Mancini, Matthew Penny, Sohrab Rahvar, Davide Ricci, Sedighe Sajadian, Jesper Skottfelt, Colin Snodgrass, John Southworth, Jeremy Tregloan-Reed, Joachim Wambsganss, Olivier Wertz, Yiannis Tsapras, Rachel A. Street, Daniel M. Bramich, Keith Horne, Iain A. Steele. An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing. Accepted to APJ, 2022 [abstract]