Critical collapse of an ultrarelativistic fluid in the Γ→1 limit

In this paper, we investigate the critical collapse of an ultrarelativistic perfect fluid with the equation of state P = (Gamma - 1)rho in the limit of Gamma -> 1. We calculate the limiting continuously self-similar (CSS) solution and the limiting scaling exponent by exploiting self-similarity of the solution. We also solve the complete set of equations governing the gravitational collapse numerically for (Gamma - 1) = 10(-2),..., 10(-6) and compare them with the CSS solutions. We also investigate the supercritical regime and discuss the hypothesis of naked singularity formation in a generic gravitational collapse. The numerical calculations make use of advanced methods such as high-resolution shock capturing evolution scheme for the matter evolution, adaptive mesh refinement and quadruple precision arithmetic. The treatment of vacuum is also nonstandard. We were able to tune the critical parameter up to 30 significant digits and to calculate the scaling exponents accurately. The numerical results agree very well with those calculated using the CSS ansatz. The analysis of the collapse in the supercritical regime supports the hypothesis of the existence of naked singularities formed during a generic gravitational collapse.