Astronomers have for the first time discovered what may be an isolated stellar-mass black hole using the astrometric microlensing technique. They estimate that the mass of the invisible object is between 1.6 and 4.4 solar masses. Because they think that the leftover remnant of a massive star must be heavier than 2.2 solar masses in order to collapse to a black hole, the researchers caution that the object could be a neutron star instead of a black hole.
The star-filled sky in this Hubble photo lies in the direction of the Galactic center. The light from stars is monitored to see if any change in their apparent brightness is caused by a foreground object drifting in front of them. The warping of space by the interloper would momentarily brighten the appearance of a background star, an effect called gravitational lensing. One such event, OB110462, is shown in the four close-up frames at the bottom. The arrow points to a star that momentarily brightened, as first captured by Hubble in August 2011. This was caused by a foreground black hole drifting in front of the star, along our line of sight. The star brightened and then subsequently faded back to its normal brightness as the black hole passed by. Because a black hole doesn’t emit or reflect light, it cannot be directly observed. But its unique thumbprint on the fabric of space can be measured through these so-called microlensing events. Image credit: NASA / ESA / Hubble / K. Sahu, STScI / J. DePasquale, STScI.
Stellar-mass black holes are produced when massive stars collapse under their own gravity.
Observations of black holes are a key ingredient for understanding outstanding questions in massive stellar evolution, such as which stars explode, which stars produce neutron stars vs. black holes, and whether there is a gap between the heaviest neutron stars and the lightest black holes.
Black holes are abundant. There are predicted to be between 10 million and one billion stellar-mass black holes in the Milky Way Galaxy alone.
However, only about two dozen have been definitively detected, all in binary systems. Beyond the Milky Way, over 80 binary black hole mergers have been detected via gravitational waves.
Isolated black holes in the Milky Way can be found and weighed using the technique of gravitational microlensing.
When a foreground lens (e.g. black hole) passes in front of a background source star, the source light is temporarily bent and split into two unresolved images by the lens mass, producing a transient signal.
The newly-detected microlensing event, designated MOA-2011-BLG-191/OGLE-2011-BLG-0462 (OB110462 for short), was caused by a wandering compact object located about 5,000 light-years away in the Carina-Sagittarius spiral arm of our Galaxy.
It was detected in the data from the NASA/ESA Hubble Space Telescope by two teams of astronomers — one led by Space Telescope Science Institute astronomer Kailash Sahu and the other by University of California, Berkeley astronomer Casey Lam.
The astrometric microlensing technique provided information on the mass, distance, and velocity of the OB110462 object.
“Astrometric microlensing is conceptually simple but observationally very tough. Microlensing is the only technique available for identifying isolated black holes,” Dr. Sahu said.
The amount of deflection by the object’s intense warping of space allowed Dr. Sahu’s team to estimate that it weighs 7 solar masses.
Dr. Lam’s team reports a slightly lower mass range, meaning that the object may be either a neutron star or a black hole. They estimate that the mass of the object is between 1.6 and 4.4 times that of the Sun.
“As much as we would like to say it is definitively a black hole, we must report all allowed solutions. This includes both lower mass black holes and possibly even a neutron star,” said University of California, Berkeley astronomer Jessica Lu.
“Whatever it is, the object is the first dark stellar remnant discovered wandering through the galaxy, unaccompanied by another star,” Dr. Lam added.
Dr. Sahu and colleagues estimate the isolated black hole is traveling across the galaxy at 160,000 kmh (100,000 mph).
“Detections of isolated black holes will provide new insights into the population of these objects in our Milky Way,” Dr. Sahu said.
The results appear in two papers to be published in the Astrophysical Journal and the Astrophysical Journal Letters.
_____
Kailash C. Sahu et al. 2022. An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing. ApJ, in press; arXiv: 2201.13296
Casey Y. Lam et al. 2022. An isolated mass gap black hole or neutron star detected with astrometric microlensing. ApJL, in press; arXiv: 2202.01903
Source link: https://www.sci.news/astronomy/microlensing-black-hole-10896.html
