“A thousand miles from all inhabited lands”, or more exactly at 5000 light years from our planet, a black hole walks alone like a cushy father. Isolated black holes aren’t considered an absolute rarity by scientists, but observing them is another pair of telescopes. Yet it is the unique feat of its kind that seems to have been achieved by a team of scientists who published their study in Astrophysical Journal (preprint). It took no less than ten years to confirm the presence of this black hole via the phenomenon known as “gravitational lensing”.
The gravitational lens is produced by the presence of a hyper massive body located between the observer and a distant light source. The gravitational field of the lens deflects the light rays to the point of modifying ultimately the image perceived by the observer. Three types of gravitational lenses are referenced, strong gravitational lenses, weak gravitational lenses and micro gravitational lenses. End of the big pedagogical parenthesis.
The gravitational lens of our solitary black hole was first seen in 2011 (MOA-2011-BLG-191/OGLE-2011-BLG-0462, cheers!). The scientists then noticed an unusual increase in the brightness of a star corresponding well to the effect of a gravitational lens. The “nature” of this lens was subsequently revealed… by elimination. A simple planet would not have been massive enough to generate the effect, a star would have been visible, etc. So there remained the black hole hypothesis, the only plausible one and “connection” with observation.