If, as astronomers believe, the death of large stars leaves black holes, there should be hundreds of millions of them scattered across 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 what could be a free-floating black hole for the first time by observing the brightening of a more distant star as its light was distorted by the strong gravitational field of the object – therefore – called gravitational microlens.
The team, led by graduate student Casey Lam and Jessica Lu, an associate professor of astronomy at UC Berkeley, estimate the mass of the unseen compact object to be between 1.6 and 4.4 times that of the Sun. Because astronomers believe the remnant of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, UC Berkeley researchers warn the object could be a neutron star instead of a black hole. Neutron stars are also dense and very compact objects, but their gravity is balanced by the 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 dim stellar remnant – a stellar “ghost” – discovered wandering the galaxy unassociated with another star.
“This is the first free-floating black hole or neutron star discovered with a gravitational microlens,” Lu said. “With the microlens, we are able to probe these solitary, compact objects and weigh them. I think we opened a new window on these dark objects, which cannot be seen otherwise.”
Determining how many of these compact objects populate the Milky Way galaxy will help astronomers understand the evolution of stars – in particular, how they die – and our galaxy, and perhaps reveal whether any of the invisible black holes are primordial black holes, which some cosmologists believe were produced in large quantities during the Big Bang.
The analysis by 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 neutron star. The team also concluded that the probable population of black holes in the galaxy is 200 million – about what most theorists had predicted.
Same data, different conclusions
Notably, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and claims that the mass of the compact object is closer to 7.1 solar masses and 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 distant star’s illumination when its light was distorted or “lensed” by the super-compact object, and astrometric measurements of the displacement of the star’s location. distant star in the sky due to gravitational force. distortion by the lens object. The photometric data comes from two microlensing surveys: 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 Experiment in Astrophysics (MOA), which is mounted on a 1.8- meter telescope in New Zealand operated by Osaka University. Astrometric data comes from NASA’s Hubble Space Telescope. STScI manages the telescope’s science program and conducts its science operations.
Since both microlensing surveys captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, for short.
While surveys like these uncover around 2,000 stars lit by microlenses each year in the Milky Way galaxy, the addition of astrometric data is what allowed the two teams to determine the mass of the compact object. and its distance from Earth. The UC Berkeley-led team estimated it to be between 2,280 and 6,260 light-years (700-1920 parsecs), toward the center of the Milky Way and near the large bulge that surrounds the black hole. central mass of the galaxy.
The STScI group estimated it to be about 5,153 light-years (1,580 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 over 100 days, and therefore could have been holes black – might not be caused by compact objects after all.
Lu, who has been searching for floating black holes since 2008, believed the data would help him better estimate their abundance in the galaxy, which has been estimated at roughly 10 million to 1 billion. To date, star-sized black holes have only been found as part of binary star systems. Black holes in binaries are visible either in X-rays, produced when material from the star falls on the black hole, or by recent gravitational wave detectors, which are sensitive to mergers 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. It’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, that would not fit our model for how many black holes there should be in the Milky Way.Something should change in our understanding of black holes – either their number, or the speed at which they move, or their mass .
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 lensing object appeared dim, and therefore not a star. ; the stellar brightening lasted a long time, nearly 300 days; and the background star position distortion was also long lasting.
The length of the lens event was the main clue, Lam said. In 2020, she showed that the best way to search for black hole microlenses was to search for very long events. Only 1% of detectable microlensing events are likely to originate 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 that last longer than 120 days are likely to be black holes.
“The duration of the brightening event is an index of the mass of the foreground lens that bends the light from the background star,” Lam said. “Long events are more likely due to black holes. This is not a guarantee, however, because the duration of the brightening episode depends not only on the mass of the foreground lens, but also on the rate at which the foreground lens and the background star are moving relative to each other.However, by also obtaining measurements of the apparent position of the background star, we can confirm whether the lens of foreground is really a black hole.”
According to Lu, the gravitational influence of OB110462 on the light from the background star was surprisingly long. It took about a year for the star to peak in 2011, then about a year to return to normal.
More data will tell 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. These new data showed that the change in position of the star resulting from the gravitational field of the lens is still observable 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’ differing conclusions are due to the fact that astrometric and photometric data give different measurements of the relative motions of foreground and background objects. The astrometric analysis also differs between the two teams. The UC Berkeley-led team argues 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 analysis. improved in the future.
“While we’d like to say it’s definitely a black hole, we should point out all allowed solutions. This includes both lower-mass black holes and possibly even a neutron star,” said said Lu.
“If you can’t believe the light curve, the brightness, then that’s telling you something important. If you can’t believe the position versus time, that’s telling you something important,” Lam said. “So if one of them is wrong, we need to figure out why. Or the other possibility is that what we’re measuring in both datasets is correct, but our model is wrong. The photometry and d astrometry comes from the same physical process, which means that brightness and position must be consistent with each other, so something is missing.
The two teams also estimated the speed of the super compact lens object. The Lu/Lam team found a relatively calm speed, less than 30 kilometers per second. The STScI team found an unusually high speed, 45 km/s, which they interpreted as the result of an extra kick the alleged black hole received from the supernova that generated it.
Lu interprets his team’s low-velocity estimate as potentially supporting a new theory that black holes are not the result of supernovae – the prevailing hypothesis today – but rather originate from failed supernovae that do not bright splashes in the universe or do not give the resulting black. hole a kick.
Lu and Lam’s work is supported by the National Science Foundation (1909641) and the National Aeronautics and Space Administration (NNG16PJ26C, NASA FINESST 80NSSC21K2043).
Letters from the Astrophysical Journal
The title of the article
An isolated black hole or mass-gap neutron star detected by astrometric microlensing