Astronomers may have discovered a ‘dark’ heat

If the death of large stars leaves black holes, as astronomers believe, hundreds of millions of them should be scattered all over the Milky Way. The problem is that isolated black holes are not visible.

Now, astronomers led by the University of California, Berkeley, have discovered for the first time what could be a free-floating black hole by observing the brightness of a distant star as the light is distorted by an object’s strong gravitational field — therefore — called microgravity.

The team is led by graduate student Casey Lam and Jessica LowAn associate professor of astronomy at the University of California, Berkeley, estimates that the mass of the invisible compact object is between 1.6 and 4.4 times the mass of the sun. Because astronomers believe that the remains of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, researchers at UC Berkeley are warning that the object could be a neutron star rather than a black hole. Neutron stars are also very dense and compact objects, but their gravity is offset by internal neutron pressure, which prevents further collapse in a black hole.

Whether a black hole or a neutron star, the object is the first dark stellar remnant — a stellar “ghost” — discovered wandering the galaxy without being associated with another star.

“This is the first floating black hole or neutron star detected by microgravity lenses,” Lu said. “Using the finer lens allows us to examine and weigh these isolated, compressed objects. I think we’ve opened a new window on these dark objects, which cannot be seen any other way.”

By determining how many of these compact objects inhabit the Milky Way, astronomers can understand the evolution of stars — specifically how they die — and the evolution of our galaxy, potentially revealing whether any of the invisible black holes are primordial black holes, which he is considering. Some cosmologists believe that large quantities were produced during the Big Bang.

The analysis of Lam, Lu and their international team has been accepted for publication in 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 concluded that the likely number of black holes in the galaxy is 200 million — roughly what most theorists had expected.

Same data, different conclusions

Notably, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event, claiming that the compact object’s mass is closer to 7.1 solar masses and an undisputed black hole. Paper describing the analysis by the STScI team led by Kailash Sahuhas been accepted for publication in Astrophysical Journal.

Both teams used the same data: photometric measurements of the brightness of a distant star as the light was distorted or “reflected” by the highly compressed object, and astronomical measurements of the changing position of the distant star in the sky due to gravity. distortion by the lens object. The optical data comes from two microlens studies: the Optical Gravitational Lens Experiment (OGLE), which uses a 1.3-meter telescope in Chile operated by the University of Warsaw, and the Microlens Observations in Astrophysics ( MOA), which is mounted on a 6-foot telescope. meter telescope in New Zealand, operated by the University of Warsaw, Osaka University. Astronomical data came from NASA’s Hubble Space Telescope. STScI manages the scientific program of the telescope and carries out the scientific activities.

Because both precision-lens explorations captured the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462 or OB110462 for short.

While studies like this one discover about 2,000 bright stars each year through microlensing in the Milky Way, it was the addition of astronomical data that allowed the two teams to determine the compact object’s mass and distance from Earth. The team, led by the University of California, Berkeley, estimated it to be located between 2,280 and 6,260 light-years (700-1920 parsecs), toward the center of the Milky Way and near the large bulge that forms the central supermassive black galaxy surrounds hole.

The STScI cluster is estimated to be about 5,153 light-years (1580 parsecs) away.

I’m looking for a needle in a haystack

Lou and Lam first became interested in the body in 2020 after the STScI team initially concluded that Five microlensing events Hubble’s observations — all of which lasted more than 100 days and could therefore be black holes — are probably not caused by compact objects at all.

Lu, who has been looking 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. So far, only star-sized black holes have been found as part of binary star systems. Black holes are seen in binary stars, either in X-rays, which are produced when material from a star falls onto a black hole, or by modern gravitational wave detectors, which are sensitive to the merger of two or more black holes. But these events are rare.

“Casey and I looked at the data and got really interested. We said, ‘Wow, there are no black holes,’” Lu said. That’s amazing, “even though it should have been there.” “And so we started looking at the data. If there really were no black holes in the data, it wouldn’t match our model of how many black holes there should be in the Milky Way. Something should change in understanding black holes – be it their number, speed or mass.”

When Lahm analyzed the photometric and astrometry of the five-minute lens events, I was surprised that one, OB110462, had the characteristics of a compact body: the lens body appeared dark and therefore not a star; stellar brightness lasted a long time, nearly 300 days; The background star position was also distorted for a long time.

Lamm said the length of the lens event was the most important tip. In 2020, it showed that the best way to search for black hole microlenses is to look for very long events. Only 1% of the minute lens events that can be detected likely come from black holes, she said, so watching all the events would be like looking for a needle in a haystack. But according to Lamm, about 40% of microlensing events lasting longer than 120 days are likely black holes.

“How long the bright event lasts is an indication of how heavily the foreground lens bends light from the background star,” Lamm said. “Longer events are most likely due to black holes. This is not a guarantee, as the duration of the bright ring depends not only on how heavy the foreground lens is, but also on how fast the foreground lens and background star move relative to. But by also taking measurements for the background star’s apparent location, we can confirm whether the foreground lens is really a black hole.”

According to Lu, OB110462’s gravitational effect on the background star’s light was surprisingly long. It took about a year for the star to light up to its peak in 2011, and then about a year to return to normal.

More data will distinguish a black hole from a neutron star

To confirm that OB110462 was the result of an extremely compact object, Low and Lam asked for more astronomical data from Hubble, some of which arrived last October. These new data showed that the change in the star’s position due to the lens’ gravitational field could still be observed 10 years after the event. More Hubble sightings of microlensing are tentatively scheduled for fall 2022.

Analysis of the new data confirmed that OB110462 was most likely a black hole or a neutron star.

Low and Lam suspect that the two teams’ different conclusions are due to the fact that the astronomical and photometric data give different measurements of the relative motions of the fore and aft objects. Astrological analysis also differs between the two teams. The UC Berkeley team says 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 in the future with more Hubble data and improved analysis.

“As much as we would definitively say it is a black hole, we must report all allowed solutions,” Lu said. “This includes lower-mass black holes and maybe even a neutron star.”

“If you can’t believe the curvature of the light, the brightness, that means something important. If you can’t believe the situation versus the time, that says something important,” Lamm said. “So, if one of them is wrong, we need to understand why. Or another possibility is that what we measure in the two data sets is correct, but our model is incorrect. The photometric and astrometric data come from the same physical process, which means that brightness and position must be consistent. With each other. So something is missing there. †

Both groups also estimate the speed of the ultra-fine lens body. The Lu/Lam team found a relatively moderate 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 given to the so-called black hole from the supernova it spawned.

Low interprets her team’s low-velocity estimate as possible support for a new theory that black holes aren’t the result of supernovae — the current assumption — but instead come from failed supernovae that don’t make a bright splash in the universe or cause the resulting blackening. 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 FINNESS 80NSSC21K2043).