image: Image of the Hubble Space Telescope of a distant star that was illuminated and distorted by an invisible but very compact and heavy object between it and Earth. The compact object, estimated by UC Berkeley astronomers to be 1.6 to 4.4 times the mass of our sun, could be a freely floating black hole, perhaps one of the 200 million in the Milky Way galaxy. see more
Credit: image courtesy of STScI / NASA / ESA
If, as astronomers believe, the death of large stars leaves behind black holes, there should be hundreds of millions scattered across the Milky Way. The problem is that the isolated black holes are invisible.
Now, a team led by the University of California, Berkeley, astronomers have discovered for the first time what a free-floating black hole might look like by observing the brightness of a more distant star when its light was distorted by the strong gravitational field. ‘object, so -call called gravitational microlenses.
The team, led by graduate student Casey Lam and Jessica Lu, an associate professor of astronomy at UC Berkeley, estimates that the mass of the invisible compact object is 1.6 to 4.4 times that of the sun. . Because astronomers think the remains of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, UC Berkeley researchers warn that the object could be a star of neutrons instead of a black hole. Neutron stars are also dense and very compact objects, but their gravity is balanced by the internal pressure of neutrons, which prevents a subsequent collapse of 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 the galaxy unmatched by another star.
“This is the first free-floating black hole or neutron star discovered with gravitational microlenses,” Lu said. “With microlenses, we can probe these solitary, compact objects and weigh them. I think we’ve opened a new window on these dark objects, which can’t be seen any other way.”
Determining how many of these compact objects populate the Milky Way will help astronomers understand the evolution of stars, in particular how they die, and our galaxy, and perhaps reveal if any of the invisible black holes are primordial black holes, which some cosmologists think they occurred in large quantities during the Big Bang.
The analysis of Lam, Lu and his international team has been accepted for publication in The Astrophysical Journal Letters. The analysis includes four other microlens events that the team concluded were not caused by a black hole, although two were likely caused by a white dwarf or a neutron star. The team also concluded that the probable population of black holes in the galaxy is 200 million, roughly what most theorists predicted.
Same data, different conclusions
In particular, a competing team from the Baltimore Institute of Space Telescope Science (STScI) analyzed the same microlens event and stated that the mass of the compact object is closer to 7.1 solar masses and, unquestionably, , has a black hole. An article describing the analysis of 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 brightness of the distant star as its light was distorted or “slowed down” by the supercompact object, and astrometric measurements of the change in the location of the distant star in the sky as a result of gravitation. distortion by the lens object. The photometric data come from two microlens 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 in Astrophysics (MOA) experiment. , 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 scientific program for the telescope and conducts its scientific operations.
Because the two microlens 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 reveal about 2,000 microlens-lit stars 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. on earth. The UC Berkeley-led team estimated that it is between 2,280 and 6,260 light-years (700-1920 parsecs) away, in the direction of the center of the Milky Way and near the large bulge surrounding the central massive black hole of the galaxy.
The STScI group estimated that it is 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 provisionally concluded that five microlens events observed by Hubble, all of which lasted more than 100 days and therefore could have black holes, may not be caused by compact objects. after all.
Lu, who has been looking for free-floating black holes since 2008, thought the data would help him better estimate its abundance in the galaxy, which has been estimated to be between 10 and 1 billion. So far, star-sized black holes have only been found as part of binary star systems. Binary black holes are seen in X-rays, produced when the star’s material falls on the black hole, or by recent gravitational wave detectors, which are sensitive to the fusion of two or more black holes. But these events are rare.
“Casey and I looked at the data and were very interested. We said, ‘Wow, there are no black holes. That’s amazing, “even though there should have been,” Lu said. “And so we started looking at the data. If there weren’t really any black holes in the data, that wouldn’t match our model of how many black holes there should be in the Milky Way. Something would have to change in the our understanding of black holes: their number or the speed with which they move or their masses “.
When Lam analyzed the photometry and astrometry of the five microlens events, he was surprised that one, OB110462, had the characteristics of a compact object: the lens object looked dark and therefore not a star. ; the stellar brightness lasted a long time, almost 300 days; and the distortion of the background star’s position was also long-lasting.
The length of the lens event was the main drawback, Lam said. In 2020, he showed that the best way to look for black hole microlenses was to look for very long events. It is likely that only 1% of detectable microlens events come from black holes, he said, so looking at all events would be like looking for a needle in a haystack. But Lam calculated that about 40 percent of microlens events lasting more than 120 days are likely to be black holes.
“The duration of the brightness event is an indication of the large amount of the foreground lens that doubles the background star’s light,” Lam said. “Long events are more likely because of black holes. It’s not a guarantee, though, because the duration of the glare episode depends not only on the massive mass of the foreground lens, but also on how quickly “The foreground lens and the background star move relatively. However, by also measuring the apparent position of the background star, we can confirm whether the foreground lens is really a black hole.”
According to Lu, the gravitational influence of OB110462 on the background star light was incredibly long. It took about a year for the star to fully light up in 2011, then about a year for it to return to normal.
More data will distinguish the black hole from the neutron star
To confirm that OB110462 was caused by a supercompact 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 as a result of the gravitational field of the lens is still observable 10 years after the event. More observations of Hubble microlens are tentatively scheduled for fall 2022.
Analysis of the new data confirmed that OB110462 was probably a black hole or a neutron star.
Lu and Lam suspect that the different conclusions of the two teams are due to the fact that the astrometric and photometric data give different measurements of the relative movements of the objects in the foreground and background. Astrometric analysis also differs between the two teams. The UC Berkeley-led team argues that it is 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 improved analysis in the future.
“As much as we’d like to say it’s definitely a black hole, we need to report all the solutions allowed. This includes both smaller-mass black holes and possibly even a neutron star,” Lu said.
“If you can’t believe the light curve, the brightness, that says something important. If you don’t believe the position based on 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. Photometry and astrometry data they arise from the same physical process, which means that brightness and position must be consistent with each other. So something is missing. “
Both teams also estimated the speed of the object of supercompact lenses. The Lu / Lam team found a …