Accretion onto a moving black hole for stiff equations of state

We revisit accretion of an asymptotically uniform fluid onto a moving Schwarzschild black hole, the relativistic analog of Bondi-Hoyle-Lyttleton accretion. Expanding on previous treatments, we focus on relativistic accretion of stiff fluids with adiabatic indices Γ≥ 5/3, which is particularly relevant for the treatment of primordial black holes captured by neutron stars. We perform numerical simulations to systematically explore the dependence of the steady-state accretion and drag rates on the asymptotic sound speed a_∞ and relative speed v_∞, and identify different regimes, both subsonic and supersonic, in which these rates either increase or decrease with respect to the spherical accretion rate for v_∞ = 0. We propose simple analytical approximations that capture the qualitative features observed in the dependence of the accretion and drag rates on a_∞ and v_∞.