General Relativistic versus Newtonian: a universality in radiation hydrodynamics

TL;DR

This paper demonstrates a universal behavior in the luminosity ratio between general relativistic and Newtonian models of accretion, depending mainly on temperature, with implications for understanding accretion physics.

Contribution

It reveals a universal ratio of luminosities in relativistic and Newtonian accretion models that depends solely on temperature, independent of gas mass fraction.

Findings

Luminosity ratio $L_{GR}/L_N$ is independent of gas mass fraction.

The ratio approaches 1 at low temperatures and increases at high temperatures.

The universality holds across various polytropic equations of state.

Abstract

We compare Newtonian and general relativistic descriptions of the stationary accretion of self-gravitating fluids onto compact bodies. Spherical symmetry and thin gas approximation are assumed. Luminosity depends, amongst other factors, on the temperature and the contribution of gas to the total mass, in both -- general relativistic ($L_{GR}$) and Newtonian ($L_N$) -- models. We discover a remarkable universal behaviour for transonic flows: the ratio of respective luminosities $L_{GR}/L_N$ is independent of the fractional mass of the gas and depends on asymptotic temperature. It is close to 1 in the regime of low asymptotic temperatures and can grow by one order of magnitude for high temperatures. These conclusions are valid for a wide range of polytropic equations of state.