Revisiting the shadow of a black hole in the presence of a plasma

Understand

We study the photon's motion around a black hole in the presence of a plasma whose density is a function of the radius coordinate by a renewed ray-tracing algorithm and investigate the influence of the plasma on the shadow of the black hole.

• The presence of plasma affects not only the size but also the shape of black hole shadow.
• Furthermore, the influence of plasma on trajectories of photons depends on the frequency of photon.
• For the high-frequency photons, the influence is negligible, on the contrary, the trajectories of low-frequency photons is affected significantly by the plasma.