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Recently neutral and charged black-hole solutions were found for static perfect fluid with the equation of state $p(r)=-rho(r)/3$, for fluid only as well as for fluid in the presence of electric field. In those works, the stability of the black holes were studied in an analytic manner, which concluded that the black holes are unconditionally unstable. In this work, we focus particularly on the {it numerical} study of the instability. For the black-hole solutions as well as the static solutions without horizons, we solve the perturbation equations numerically and find the unstable mode functions.
Binary black holes with spins that are aligned with the orbital angular momentum do not precess. However, post-Newtonian calculations predict that up-down binaries, in which the spin of the heavier (lighter) black hole is aligned (antialigned) with t
We investigate black holes formed by static perfect fluid with $p=-rho/3$. These represent the black holes in $S_3$ and $H_3$ spatial geometries. There are three classes of black-hole solutions, two $S_3$ types and one $H_3$ type. The interesting sol
We numerically study the superradiant instability of charged massless scalar field in the background of charged stringy black hole with mirror-like boundary condition. We compare the numerical result with the previous analytical result and show the d
We investigate the gravitational field of static perfect-fluid in the presence of electric field. We adopt the equation of state $p(r)=-rho(r)/3$ for the fluid in order to consider the closed ($S_3$) or the open ($H_3$) background spatial topology. D
We study a spherically symmetric spacetime made of anisotropic fluid of which radial equation of state is given by $p_1 = -rho$. This provides analytic solutions and a good opportunity to study the static configuration of black hole plus matter. For