An ordinary black hole is surrounded by a thin photon sphere — a place where light can loop around before falling in or escaping. In the new work, this region has puffed up into a whole layer packed with massless particles that zip around at nearly the speed of light and don't press against the walls. That's how the image of a black hole in a thick fur coat of ultrarelativistic gas was born.
The authors took spherically symmetric solutions to Einstein's equations and added a shell to the black hole — a swarm of particles on stable orbits. The result is an object they've dubbed the 'hillingar' (from the Icelandic for 'mirage').
To a distant observer, a hillingar looks just like a bare black hole of the same mass. No difference in shadows or radiation — optical indistinguishability.
Why does this matter? If such objects can exist in nature, some of the black holes we know might actually be 'dressed'. Testing this is hard, but the authors are already working on calculations of thermodynamics and stability of hillingars — maybe there we'll find keys to observational tests.
The name 'hillingar' comes from Icelandic folklore: it referred to phantom mirages that deceive travelers — a perfect metaphor for a black hole pretending to be ordinary.
In Stanisław Lem's novel 'Solaris', the ocean creates phantom objects indistinguishable from real ones — hillingars play the same game with astronomers.