A lantern shines through thick fog. Light scatters off water droplets, and the lantern appears blurred. Something similar happens around a black hole: hot gas, like fog, envelops it, while X-rays from the inner parts of the disk struggle through this cloud. Only here, the "droplets" are electrons, and the "fog" itself is called the corona.
The authors of the new study built a model of such a corona for a black hole that is accreting matter very slowly — at about one-hundredth of the maximum possible rate. They considered not only light scattering off electrons but also pair production: when two photons collide with sufficient energy, they can transform into an electron and its antiparticle, a positron. This process, it turns out, significantly densifies the corona and squeezes it closer to the black hole.
The model allowed the team to compute how the X-ray emission would appear to an observer at different angles. It turns out that the degree of polarization (how much the light waves oscillate in a single plane) in the 2–8 keV range reaches 4–10%. These numbers agree well with data from real binary systems where a black hole pulls matter from a companion star. Such an "X-ray lantern" helps us peer into the very heart of an accreting black hole.
Electron-positron pairs in a black hole's corona are born from pure energy — two photons collide and vanish, and in their place appear two particles with mass. This is a direct example of Einstein's E=mc² in action.
In Charles Stross's novel "Accelerando," civilizations are described that use black hole accretion disks as sources of energy and raw materials for producing antimatter — almost like in this model.