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Abstract Models of highly sub-Eddington accretion onto black holes commonly use a single-fluid model for the collisionless, near-horizon plasma. These models must specify an equation of state. It is common to use an ideal gas withp = (γ − 1)uandγ = 4/3, 13/9, or 5/3, but these produce significantly different outcomes. We discuss the origins of this discrepancy and the assumptions underlying the single-fluid model. The main result of this investigation is that under conditions relevant to low-luminosity black hole accretion the best choice of single-fluid adiabatic index is close to but slightly less than 5/3. Along the way we provide a simple equilibrium model for the relation between the ion-to-electron dissipation ratio and the ion-to-electron temperature ratio, and explore the implications for electron temperature fluctuations in Event Horizon Telescope sources.
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Relativistic Bondi accretion for stiff equations of state
ABSTRACT We revisit Bondi accretion – steady-state, adiabatic, spherical gas flow on to a Schwarzschild black hole at rest in an asymptotically homogeneous medium – for stiff polytropic equations of state (EOSs) with adiabatic indices Γ > 5/3. A general relativistic treatment is required to determine their accretion rates, for which we provide exact expressions. We discuss several qualitative differences between results for soft and stiff EOSs – including the appearance of a minimum steady-state accretion rate for EOSs with Γ ≥ 5/3 – and explore limiting cases in order to examine these differences. As an example, we highlight results for Γ = 2, which is often used in numerical simulations to model the EOS of neutron stars. We also discuss a special case with this index, the ultrarelativistic ‘causal’ EOS, P = ρ. The latter serves as a useful limit for the still undetermined neutron star EOS above nuclear density. The results are useful, for example, to estimate the accretion rate on to a mini-black hole residing at the centre of a neutron star.
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- PAR ID:
- 10232903
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 502
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 3003 to 3011
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/stab161
