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

Abstract

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. Interestingly, it is also found that the black hole image would take on a multi-ring structure due to the presence of plasma.

Calculating black hole shadows: Review of analytical studies

“…Several examples are worked out in the paper by Perlick and Tsupko [233], and several others, again for cases where the separability condition is satisfied, in a follow-up paper by Yan [234]. For some examples where the separability condition is not satisfied, the shadow was numerically calculated by Huang, Dong and Liu [235]. If the plasma frequency is not small in comparison to the frequency of the light, the shape of the shadow can be significantly different from the vacuum case.…”

Shadow of a black hole surrounded by dark matter

“…where K = 12∆Mr s ∆r s (∆r s − 3M ) (26) −18M ∆M∆r 2 s + 3∆Mr 2 s (4∆r s + 3∆M) + ∆r 4 s . There are two photon spheres: the other one is given by the same equation (25), where K comes with an opposite sign in eq.…”

Influence of a plasma on the observational signature of a high-spin Kerr black hole

“…The complete signature at EHT should be the combined information of the black hole shadow and the signal from the hot spot. In addition to the influence on the size and shape of a black hole discussed in [25,27], we find that there is a special segment of the shadow edge originating from the nearhorizon region and is approximately the same for both of the power-law-like models (18) and (19). Moreover, the image position and redshift of the hot spot are obviously influenced by the plasma as well.…”

“…The shadow for a Kerr black hole in a plasma has been studied in Refs. [23,25,27]. However, it is worth to revisit this for a high-spin black hole since doing so helps one to understand the image of a hot spot (and thus, the signature at EHT) better.…”