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To approach a more reliable observational signature of a high-spin Kerr blackhole, one should take into account the effects of its surroundings. To this endwe study in this paper the influence of a surrounding plasma. We consider itsrefractive and dispersive effects on photon trajectories and ignore thegravitational effects of plasma particles as well as the absorption orscattering processes of photons. With two specific plasma models, we obtainanalytical formulae for the black hole shadow and for the observationalquantities of an orbiting "hot spot" seen by an observer located far away fromthe black hole. We find that the plasma has a frequency-dependent dispersiveeffect on the size and shape of the black hole shadow and on the image positionand redshift of the hot spot. These results may be tested by the Event HorizonTelescope in the future.

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