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 $\mathrm{\Gamma }\ge 5\mathrm{/}3$, 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 $a_{\mathrm{\infty }}$ and relative speed $v_{\mathrm{\infty }}$, and identify different regimes, both subsonic and supersonic, in which these rates either increase or decrease with respect to the spherical accretion rate for $v_{\mathrm{\infty }}=0$. We propose simple analytical approximations that capture the qualitative features observed in the dependence of the accretion and drag rates on $a_{\mathrm{\infty }}$ and $v_{\mathrm{\infty }}$.