• Accepted Paper

Accretion onto a moving black hole for stiff equations of state

Elliott Ewell, Peter K. Harris, Thomas W. Baumgarte, and Stuart L. Shapiro

Phys. Rev. D - Accepted 22 October, 2026

DOI: https://doi.org/10.1103/xl58-tn9c

Abstract

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 /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∞.

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