Astronomers have validated the first detection of what appears to be a wandering black hole drifting through the Milky Way. The research has been accepted into The Astrophysical Journal and is available on arXiv.
Small stellar-mass black holes form during the gravitational collapse of massive stars. Searching for and determining the properties of these objects is important both for defining the boundary between neutron stars and black holes, and for testing models of the final evolution of massive stars.
It is known that there are between 107 and 109 stellar-mass black holes in the Milky Way. However, there are only 20 reliable cases of detection, all part of binary systems.
Isolated black holes
According to astrophysicists, many stellar-mass black holes would be represented by unique objects. This is because about 30% of massive stars form as single stars. And in cases of closed binary systems, stars can merge before a supernova explosion.
In a large binary system, the impulse imparted to one of the components during the explosion of another star as a supernova can be so great that it would be capable of destroying the system, leaving an isolated black hole.
Isolated black holes are extremely difficult to detect, since they are not sources of their own radiation. Furthermore, its rate of accretion of matter from the interstellar medium is too low to generate perceptible radio or X-ray emission.
Microlensing turns out to be the only method available to measure the mass of isolated black holes. This astronomical phenomenon occurs when a star or compact object that acts as a lens passes in front of a star that acts as a radiation source.
Now, a team of astronomers led by Casey Lam of the University of California, Berkeley has reported a new case of measuring the mass of an isolated stellar-mass black hole using gravitational microlensing.
Scientists analyzed five black hole candidates found in sky surveys for OGLE and MOA gravitational microlensing events. Then, in 2011, they identified microlensing event MOA-11-191/OGLE-11-0462 (abbreviated OB110462) among them, observed near the center of the Milky Way.
The photometric light curves for each candidate cover a period of 7 to 11 years. Astrometric data were obtained by the Hubble Space Telescope and covered each microlensing event.
The astrometric shift of the background star was determined to be more than a milliarcsecond. Likewise, the mass of the OB110462 lens object is in the range of 1.6 to 4.4 solar masses. This makes it the first case of detection of a compact object with the use of astrometric microlensing.
A second study
However, shortly before submitting the completed scientific paper for review, researchers learned of an independent analysis of event OB110462. This had been done by Kailash Sahu’s team, who estimated the mass of the lensed object to be 7.1 solar masses, suggesting that it is a stellar-mass black hole.
It is not yet clear whether this discrepancy is due to the use of different volumes of observational data. The difference may also be due to different data analysis techniques.
A preliminary study shows that the different choice of reference stars in the two documents is not a source of inconsistency. Much more work will be needed to fully understand what is going on.