Astronomers may have discovered ‘dark’ heat

If the death of a large star left a black hole, as astronomers believe, there should be hundreds of millions of them scattered throughout the Milky Way. The problem is that isolated black holes are invisible.

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

The team is led by graduate students Casey Lam and Jessica LowA professor of astronomy at the University of California, Berkeley, estimates that the mass of the invisible solid 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, UC Berkeley researchers warned 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 balanced by the internal neutron pressure, which prevents further collapse into the black hole.

Whether it’s a black hole or a neutron star, the object is the remnant of the first dark star – a stellar “ghost” – to be found wandering in a galaxy unrelated to any other star.

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

Determining how much dense matter inhabits the Milky Way will help astronomers understand the evolution of stars — specifically, how they died — and the evolution of our galaxy, which may reveal whether any invisible black holes were primordial black holes, which he considers. large quantities were produced during the Big Bang.

Analysis by Lam, Lu and their international team has been accepted for publication in Astrophysics Journal Letter. The analysis included four other microlensing events that the team concluded were not caused by black holes, although two were likely caused by white dwarfs or neutron stars. The team also concluded that the possible number of black holes in the galaxy is 200 million – as most theorists expect.

Same data, different conclusions

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

Both teams used the same data: photometric measurements of the brightness of distant stars as their light is distorted or “reflected” by very dense objects, and astronomical measurements of changes in the position of distant stars in the sky as a result of gravity. distortion by the lens object. The optical data came from two microlens surveys: the Optical Gravity Lens Experiment (OGLE), which used a 1.3-meter telescope in Chile operated by the University of Warsaw, and Microlens observations in Astrophysics (MOA), which was mounted on the lens of a 1.8 telescope. -meters in New Zealand operated by the University of Warsaw University of Osaka. The astronomical data comes from NASA’s Hubble Space Telescope. STScI manages the telescope science program and conducts its science operations.

Because both precision surveillance lenses capture the same object, it goes by two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462 for short.

While surveys like this one find about 2,000 bright stars with micro-lens each year in the Milky Way, the addition of astronomical data allowed the two teams to determine the mass and distance of the compact object from Earth. The team, led by the University of California, Berkeley, estimate that it lies between 2,280 and 6260 light-years (700-1920 parsecs), toward the center of the Milky Way and near the large bulge that surrounds the center of the supermassive black galaxy. hole.

The STScI cluster is estimated to be about 5,153 light-years away (1,580 parsecs).

I’m looking for a needle in a haystack

Lou and Lam first became interested in bodies in 2020 after the STScI team initially concluded that Five microlensing events Those observed by Hubble – all of which have been going on for more than 100 days, and therefore could be black holes – may not be caused by solid bodies at all.

Lu, who has been searching for free-moving black holes since 2008, thinks the data will help him better estimate their abundance in the galaxy, which is roughly estimated to be between 10 million and 1 billion. So far, only stellar-sized black holes have been found as part of binary star systems. Black holes are seen in binary either in X-rays, which are produced when material from stars fall into a black hole, or by modern gravitational wave detectors, which are sensitive to the merging 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, no black holes,’” Lu said. That’s awesome, “even though there should be.” “So, we started looking at the data. If there really were no black holes in the data, this would not fit our model of how many black holes there should be in the Milky Way. Something must change in understanding black. holes — either their number, their speed, or their mass.”

When Lahm analyzed the photometric and event astrometry of the five-minute lenses, I was surprised that one, the OB110462, had the characteristics of a compact body: the lens body appeared dark, and therefore not stellar; star brightness lasts a long time, almost 300 days; Background star position distortion is also long term.

Lamm says the length of the lens event is a key tip. In 2020, it was shown that the best way to search for black hole microlenses is to search for very long events. Only 1% of detectable minute lens events likely originate from black holes, he says, so seeing all of those events is like looking for a needle in a haystack. However, according to Lamm, about 40% of microlensing events lasting more than 120 days are most likely black holes.

“How long the bright event lasts is an indication of how much the foreground lens bends the background starlight,” Lamm said. “Longer events are most likely caused by black holes. This is not a guarantee, as the duration of the ring of light depends not only on how large the foreground lens is, but also on how fast the foreground lens and background stars are moving relative to However, by also obtaining measurements for the apparent location of the background stars, we can determine whether the foreground lens is really a black hole.”

According to Lu, the gravitational effect of OB110462 on the background starlight is very long. It took about a year for the star to peak in 2011, and then about a year to return to normal.

More data will distinguish black holes from neutron stars

To confirm that OB110462 resulted from a very compact object, Low and Lam requested more astronomical data from Hubble, some of which arrived last October. This new data shows that changes in the position of stars due to the gravitational field of the lens can still be observed 10 years after the event. More Hubble observations of microlensing are tentatively slated for fall 2022.

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

Low and Lam suspect that the two teams’ differing conclusions are due to the fact that the astronomical and photometric data provide different measures of the relative motion of the object forward and backward. Astrological analysis also differed between the two teams. The UC Berkeley team thinks it’s not yet possible to tell whether the object is a black hole or a neutron star, but they hope to resolve the discrepancy with more Hubble data and better analyzes in the future.

“As much as we say for sure it’s a black hole, we have to report all possible solutions. This includes a lower mass black hole and maybe even a neutron star,” Lu said.

“If you can’t believe the curve of light, brightness, it means something important. If you can’t believe the situation versus time, it tells you something important,” Lamm said. “So, if one of them is wrong, we have to understand the reason. Or another possibility is that what we measure in the two data sets is correct, but our model is wrong. Photometric and astrometric data come from the same physical process, meaning brightness and position must be consistent. One another. So there’s something missing there.”

Both groups also estimated the ultrafine lens body speed. Lu/Lam’s team found a relatively moderate speed, less than 30 kilometers per second. The STScI team discovered the unusually high speed of 45 km/s, which they interpreted as the result of an extra kick called a black hole from the resulting supernova.

Low interprets his team’s low velocity estimates as possible support for a new theory that black holes are not the result of supernovae – the current assumption – but are from failed supernovae that did not make a bright spark in the universe or produce results. black hole kick.

Lu and Lam’s work was supported by the National Science Foundation (1909641) and the National Aeronautics and Space Administration (NNG16PJ26C, NASA FINNESS 80NSSC21K2043).