Self-Similar Hot Accretion Flow onto a Neutron Star

We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. We show that the accretion flow has an extended settling region which can be described by means of two analytical self-similar solutions: a two-temperature solution which is valid in an inner zone, r . 102.5, where r is the radius in Schwarzchild units; and a one-temperature solution which is valid in an outer zone, r & 102.5. In both zones the density varies as ρ ∝ r and the angular velocity as Ω ∝ r−3/2. We solve the flow equations numerically and confirm that the analytical solutions are accurate. The self-similar settling solution differs from the advection-dominated accretion flow discussed in the context of black hole accretion. The settling flow radiates the energy dissipated by viscosity; so it is not advection-dominated. Except for the radial velocity, all other gas properties — density, angular velocity, temperature, luminosity, angular momentum flux — are independent of the mass accretion rate; these quantities do depend sensitively on the spin of the neutron star. The angular momentum flux is outward under most conditions; therefore, the central star is nearly always spun-down. The luminosity of the settling zone arises from the rotational energy that is released as the star is braked by viscosity, and the contribution from gravity is small; hence the radiative efficiency, η = Lacc/Ṁc , can be arbitrarily large at low Ṁ . For reasonable values of the gas adiabatic index γ, the Bernoulli parameter is negative; therefore, in the absence of dynamically important magnetic fields, a strong outflow or wind is not expected. The flow is convectively stable. Subject headings: accretion, accretion disks — stars: neutron