Effects of matter with anisotropic pressure on the Fan–Wang regular black hole shadows

Abstract

In this work, we investigate the consequences of an exotic fluid exhibiting negative radial and tangential pressures—thereby violating the Zel’dovich limit—on a regular solution that generalizes the Schwarzschild black hole. Specifically, we focus on the regular Fan–Wang spacetime and analyze how the presence of such a fluid modifies the black hole shadow images through negative equations of state for both pressure components. Although fundamentally different from quintessence, we consider constant radial and tangential state parameters to emulate, but not reproduce, the effects of dark energy. Furthermore, we explore the main properties of infalling spherical accretion flows and study how these state parameters influence the horizons, photosphere, and impact parameter of the Fan–Wang black hole.  We determine the black hole shadow by analyzing the null geodesics of massless test particles in the background spacetime, which serves as a reference geometry. The physical photon shadow is obtained using the effective metric induced by nonlinear electrodynamics. We examine the observed intensity under two spherical accretion scenarios in both geometries. Finally, we provide a physical interpretation of the role of negative pressures in our results and discuss possible extensions of this framework to the isotropic case.