Test particle motion around a black hole dressed with a spherically symmetric stationary fluid

Abstract

We investigate the motion of a massive particle around a spherically symmetric black hole surrounded by a stationary and radial inflow of perfect fluid. The background spacetime is modelled as a spherically symmetric solution to the Einstein field equations, where the effect of the fluid on the geometry is treated as a perturbation on the Schwarzschild background. The equation of state for the fluid is assumed to follow the linear relationship $p = w ρ$, where $p$ is the pressure, $ρ$ is the energy density with $w$ being a constant. The stress-energy tensor is treated as a phenomenological model to capture deviations from the vacuum Einstein theory. We allow the parameter $w$ of the equation of state to take both positive and negative values accepting a broad range of scenarios including exotic ones. Specifically, we examine the cases $w =2/3$, $1/3$, $-3/4$ and $-4/3$. For $ρ\geq0$, the former two cases satisfy all standard energy conditions while the case of $w=-3/4$ violates the strong energy condition and the case of $w=-4/3$ violates all standard energy conditions. By solving the geodesic equations, we visualize the time-like geodesics around the black hole, focusing on the apsis shift of the orbit. To gain further insight into the effects of accretion, we employ the method of osculating orbital elements. Additionally, we analyze the observable effects on spacetime by studying the redshift of the orbiting test particles as an example of possible observables. We show that the difference in the particle orbits due to the matter accretion may be probed by using the redshift observation of stars orbiting around the black hole.