A compact object appears to be wandering around our galaxy. This could be the first detection of a solitary wandering black hole inside the Milky Way.
And black hole isolated may have finally been spotted in our galaxy. Two teams of scientists relied on data from the Hubble Space Telescope (the pre-publications of their work are available online, ici et to be): although their results are different, the two teams suggest the presence of a compact object, summarize a press release from NASA on June 10, 2022.
« We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole », we read in one of the two studies. Scientists already know that 100 million black holes must be roaming the Milky Way, untethered, but identifying one of these isolated specimens has never before been possible. Now we know more about this very good candidate. The potential black hole studied here is about 5,000 light-years from Earth. It would move at the speed of 160,000 km/h (we could travel from Earth to the Moon in less than 3 hours at this speed).
By definition, black holes are optically invisible: telescopes cannot “see” them, since they do not emit light. Black holes can be identified through their interactions with other objects: this is often the case for stellar-mass black holes accompanied bystars. This is also what would make the isolated specimen spotted a more unusual case.
The light from the star was amplified for 270 days by the black hole
On the other hand, the presence of black holes has the effect of deflecting or amplifying the light emitted by other celestial bodies, such as stars, when these are momentarily aligned with the black hole. This is the phenomenon of gravitational lensing (and here more exactly, of gravitational microlens). Telescopes can monitor the brightness of stars and spot when their brightness changes, if a massive object comes between a star and us. If this massive object is a black hole, scientists will be able to find out, because this event has very specific characteristics. We thus know that, under the effect of the gravity of the black hole, the gravitational microlensing event is extended by more than 200 days.
It is thanks to this technique that the scientists were able to obtain information such as the mass and the distance of the probable wandering black hole. The passage of the compact object in front of a star located in the background has been detected. The light from the star was amplified for 270 days. 6 years of observations with Hubble have made it possible to follow this deviation precisely.
It would be between 1.6 and 4.4 times as massive as the sun, according to one of the teams of scientists. The other team rather estimates that the black hole is 7 solar masses. This difference in results is not trivial. If the object is in the lower part of the estimate of 1.6 to 4.4 solar masses, we cannot exclude that it is rather a neutron star, instead of a black hole. It would therefore be necessary to eliminate this scenario before asserting that it is indeed a black hole.