An article published in the journal “Science” reports the discovery of what could be the lowest-mass known black hole, perhaps even the first of a new class of black holes. A team of researchers led by Professor Todd Thompson of Ohio State University used data from APOGEE (Apache Point Observatory Galactic Evolution Experiment) to discover a binary system that includes an invisible object that could be a black hole. If instead it were a neutron star it would be by far the most massive known, far beyond the theoretical highest limit before the collapse of such an object into a black hole.
Black holes and neutron stars are two possible ways for a star to end its life after exploding into a supernova. According to theoretical models, if the remaining core has a mass less than 2.5 times the Sun’s it becomes a neutron star, beyond that the mass collapses into a black hole. However, the known neutron stars have a typical mass that is 2.1 times the Sun’s while the lowest-mass known black holes have a mass between 5 and 6 times the Sun’s. In the gap between those masses our knowledge is still theoretical but Todd Thompson’s team tried to find some objects in that category.
This research started from the data of the APOGEE survey, which collected the light spectra of over 100,000 stars in the Milky Way. Those spectra can reveal the presence of a star’s invisible companion that causes its periodic movements having among the consequences an alteration of its light emissions. The researchers selected 200 stars that could be in that situation and their data were crossed with those of another survey, ASAS-SN (All-Sky Automated Survey for Supernovae). The result was the identification of the red giant star 2MASS J05215658+4359220, or simply J05215658, which seemed to be orbiting an invisible companion.
The researchers added more data collected by the TRES (Tillinghast Reflector Echelle Spectrograph) instrument at the FLWO (Fred Lawrence Whipple Observatory) and ESA’s Gaia space probe. All the available data made it possible to calculate the characteristics of the two objects, which orbit each other in about 83 days. The image (Courtesy Ohio State by Jason Shults. All rights reserved) shows an artistic concept of black hole – bottom left – near a red giant.
The mass of J05215658’s invisible companion was the most important result and caused the surprise because the probability peak of the calculated value is 3.3 times the Sun’s. Even with so much data available, the results have a margin of error far from small since the mass of the invisible object ranges from a minimum of 2.6 to a maximum of 6.1 times the Sun’s. The values at the extremes are unlikely but can’t be ruled out, making evaluations difficult.
The invisible object could be a very massive neutron star but usually these objects have strong electromagnetic emissions. The most likely hypothesis is that it’s a relatively low-mass black hole. In this case, it’s of the non-interactive type, which means that it doesn’t steal gas from its companion and isn’t surrounded by materials that can be heated and emit electromagnetic radiations.
Todd Thompson commented that his team offered a new way to look for black holes but also potentially identified one of the first in a new class of low-mass black holes that astronomers didn’t know before. Object masses tell us about their formation and evolution, and tell us about their nature. Only by verifying the existence of these black holes can we obtain a complete census of these extreme objects.