Astronomers May Have Spotted A Dark Free F

Credit: Image courtesy of STScI / NASA / ESA If astronomers believe that the death of large stars leaves black holes behind, there must be hundreds of millions of them scattered throughout the galaxy. The problem is that the individual 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 be by observing the glow of a more distant star as its light was distorted by the object’s strong gravitational field. -is called a gravitational microfiche. 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 solid is 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 that the object could be a neutron star instead of a black hole. Neutron stars are also dense, extremely solid objects, but their gravity is balanced by the internal neutron pressure, which prevents further collapse in a black hole. Whether it is a black hole or a neutron star, the object is the first dark stellar remnant – a stellar “ghost” – to be discovered wandering in the galaxy without coupling to another star. “This is the first free-floating black hole or neutron star to be discovered with a gravitational microscope,” Lou said. “With microfiches, we are able to detect these lonely, compact objects and weigh them. I think we have opened a new window on these dark objects, which do not look any other way.” Determining how many of these solid objects live in the Galaxy will help astronomers understand the evolution of stars — especially how they die — and our own galaxy, and perhaps reveal if any of the invisible black holes are primordial black holes. Cosmologists believe that they were produced in large quantities during the Big Bang. The analysis by Lam, Lu and their international team was accepted for publication in The Astrophysical Journal Letters. The analysis includes four other microfiche events that the team concluded were not caused by a black hole, although two were most likely caused by a white dwarf or a neutron star. The team also concluded that the potential population of black holes in the galaxy is 200 million – roughly what most theorists have predicted. Same data, different conclusions In particular, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same micro-lens event and claimed that the mass of the solid object is closer to 7.1 solar masses and undoubtedly a black hole. A paper describing the analysis by the STScI team, led by Kailash Sahu, was accepted for publication in The Astrophysical Journal. Both groups used the same data: photometric measurements of the bright star’s luminosity as its light was distorted or “lensed” by the extremely solid object, and astrometric measurements of the distant star’s displacement in the sky as a result of gravitational distortion from the object lens. The photometric data came from two microscope investigations: the Optical Gravitational Lens 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 located at a 1.8-meter telescope in New Zealand operated by Osaka University. The astronomical data came from NASA’s Hubble Space Telescope. STScI manages the scientific program for the telescope and performs its scientific functions. Because both microscope surveys caught the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, for short. While research like this reveals about 2,000 microscopic stars illuminated each year in the Galaxy, the addition of astronomical data is what allowed the two groups to determine the mass of the solid object and its distance from Earth. UC Berkeley’s team estimated it to be between 2,280 and 6,260 light-years (700-1920 parsecs) away, toward the center of our galaxy and close to the large bulge that surrounds the galaxy’s central black hole. The STScI team estimated it is about 5,153 light-years (1,580 parsecs) away. I’m looking for a needle in a haystack Lou and Lam became interested in the subject in 2020 after the STScI team concluded that five Hubble microfiche events – all of which lasted more than 100 days and could therefore be black holes – may not be caused by solid objects despite all this. Lou, who has been searching for freely floating black holes since 2008, believed the data would help her better estimate their abundance in the galaxy, which is estimated to be roughly between $ 10 billion and $ 1 billion. To date, star-sized black holes have only been found as part of binary star systems. Black holes in binaries are seen either on X-rays, which are produced when material from the star falls into the black hole, or by recent gravitational wave detectors, which are sensitive to mergers of two or more black holes. But these facts are rare. “Casey and I saw the data and we were very interested. We said, ‘Wow, no black holes.’ That’s amazing, “though they should be,” Lou said. “And so, we started looking at the data. If there really were no black holes in the data, then that would not match our model of how many black holes there should be in our Galaxy. Something would have to change in our understanding. of black holes – either their number or how fast they move or their masses “. When Lam analyzed photometry and astrometry for the five microfiche events, he was surprised that one, OB110462, had the characteristics of a solid object: The lens object looked dark, and therefore not a star. The stellar glow lasted a long time, almost 300 days. and the deformation of the background star position was also long-lasting. The duration of the lens event was the main information, Lam said. In 2020, it showed that the best way to look for micro-lens black holes was to look for very large events. Only 1% of detectable microfiche events are likely to come from black holes, he said, so looking at all the facts would be like looking for a needle in a haystack. But, Lam estimated, about 40% of microfiche events lasting more than 120 days are likely to be black holes. “How long the flash event lasts is a hint of how massive the foreground lens is that bends the starlight in the background,” Lam said. “Big events are more likely due to black holes. However, this is not a guarantee, because the duration of the brightness episode depends not only on the mass of the foreground lens, but also on how fast the foreground lens and the star move.” However, by also taking measurements of the apparent position of the background star, we can confirm whether the foreground lens is really a black hole. “ According to Lu, the gravitational effect of OB110462 on the background star light was surprisingly large. It took about a year for the star to shine at its peak in 2011 and 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 an extremely solid object, Lu and Lam requested more astronomical data from Hubble, some of which arrived last October. These new data showed that the change in star position as a result of the lens’s gravitational field is still observable 10 years after the event. Further Hubble microscope observations are temporarily scheduled for fall 2022. Analysis of the new data confirmed that OB110462 was most likely a black hole or a neutron star. Lu and Lam suspect that the different conclusions of the two groups are due to the fact that astronomical and photometric data give different measurements of the relative motions of objects in the foreground and background. Astrometric analysis also differs between the two groups. UC Berkeley’s team says it is not yet possible to tell if the object is a black hole or a neutron star, but hopes to resolve the dispute with more Hubble data and improved resolution in the future. “As much as we would like to say that it is definitely a black hole, we have to list all the allowed solutions. This includes both smaller mass black holes and possibly even a neutron star,” Lu said. “If you can not believe the light curve, the brightness, then that says something important. If you do not believe the position in relation to time, that’s telling you something important,” Lam said. “Well, if one of them is wrong, we need to understand why. Or the other possibility is that what we measure in both data sets is right, but our model is wrong. Photometry and astrometry data come from the same physical process. “which means that the brightness and the position must be consistent with each other. So something is missing there.” Both groups also appreciated 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, …