An accurate Newtonian description of particle motion around a Schwarzschild black hole

TL;DR

This paper introduces a new generalized Newtonian potential derived from Schwarzschild spacetime that accurately reproduces key relativistic features of particle motion, improving modeling of accretion disks and tidal disruptions with minimal computational cost.

Contribution

A novel generalized Newtonian potential that precisely mimics relativistic particle dynamics around Schwarzschild black holes, enhancing astrophysical simulations.

Findings

Reproduces exact locations of key orbits and frequencies within 6%.

Matches relativistic energy flux and efficiency within 7%.

Agrees well with relativistic simulations of tidal disruptions.

Abstract

A generalized Newtonian potential is derived from the geodesic motion of test particles in Schwarzschild spacetime. This potential reproduces several relativistic features with higher accuracy than commonly used pseudo-Newtonian approaches. The new potential reproduces the exact location of the marginally stable, marginally bound, and photon circular orbits, as well as the exact radial dependence of the binding energy and the angular momentum of these orbits. Moreover, it reproduces the orbital and epicyclic angular frequencies to better than 6%. In addition, the spatial projections of general trajectories coincide with their relativistic counterparts, while the time evolution of parabolic-like trajectories and the pericentre advance of elliptical-like trajectories are both reproduced exactly. We apply this approach to a standard thin accretion disc and find that the efficiency of energy extraction agrees to within 3% with the exact relativistic value, while the energy flux per unit area as a function of radius is reproduced everywhere to better than 7%. As a further astrophysical application we implement the new approach within a smoothed particle hydrodynamics code and study the tidal disruption of a main-sequence star by a supermassive black hole. The results obtained are in very good agreement with previous relativistic simulations of tidal disruptions in Schwarzschild spacetime. The equations of motion derived from this potential can be implemented easily within existing Newtonian hydrodynamics codes with hardly any additional computational effort.