Shadow of Kottler black hole in the presence of plasma for a co-moving observer

Abstract: We consider a spherically symmetric black hole metric in (3+1)-dimensions in presence of a positive cosmological constant $\Lambda$. We use a general approach as proposed in \cite{1} to transform the metric in co-moving coordinates. Then for a general metric we have evaluated the angular shadow size both in absence and presence of plasma. By using the aberration relation, we then find the corresponding shadow radius as viewed by a co-moving observer for Schwarzschild de-Sitter or Kottler spacetime. The effect of cosmological constant $\Lambda$ and plasma parameter $k$ on the angular size of black hole shadow has been observed in detail. Finally, by using the observed angular size of M87$<sup>*$ and Sgr A$<sup>*$ black hole, we constrain the value of the plasma parameter $k$ with a specific observational value of the cosmological constant $\Lambda$ (or the Hubble constant $H_0$).