Accretion onto a black hole in a string cloud background
Abstract
We examine the accretion process onto the black hole with a string cloud background, where the horizon of the black hole has an enlarged radius ${r}_{H}=2M/(1\ensuremath{-}\ensuremath{\alpha})$, due to the string cloud parameter $\ensuremath{\alpha}$ ($0\ensuremath{\le}\ensuremath{\alpha}<1$). The problem of stationary, spherically symmetric accretion of a polytropic fluid is analyzed to obtain an analytic solution for such a perturbation. Generalized expressions for the accretion rate $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{M}$, critical radius ${r}_{s}$, and other flow parameters are found. The accretion rate $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{M}$ is an explicit function of the black hole mass $M$, as well as the gas boundary conditions and the string cloud parameter $\ensuremath{\alpha}$. We also find the gas compression ratios and temperature profiles below the accretion radius and at the event horizon. It is shown that the mass accretion rate, for both the relativistic and the nonrelativistic fluid by a black hole in the string cloud model, increases with increase in $\ensuremath{\alpha}$.