  In this work we revisit the steady state, spherically symmetric gas accretion
problem from the non-relativistic regime to the ultra-relativistic one. We
first perform a detailed comparison between the Bondi and Michel models, and
show how the mass accretion rate in the Michel solution approaches a constant
value as the fluid temperature increases, whereas the corresponding Bondi value
continually decreases, the difference between these two predicted values
becoming arbitrarily large at ultra-relativistic temperatures. Additionally, we
extend the Michel solution to the case of a fluid with an equation of state
corresponding to a monoatomic, relativistic gas. Finally, using general
relativistic hydrodynamic simulations, we study spherical accretion onto a
rotating black hole, exploring the influence of the black hole spin on the mass
accretion rate, the flow morphology and characteristics, and the sonic surface.
The effect of the black hole spin becomes more significant as the gas
temperature increases and as the adiabatic index $\gamma$ stiffens. For an
ideal gas in the ultra-relativistic limit ($\gamma=4/3$), we find a reduction
of 10 per cent in the mass accretion rate for a maximally rotating black hole
as compared to a non-rotating one, while this reduction is of up to 50 per cent
for a stiff fluid ($\gamma=2$).

High Energy Astrophysical Phenomena