Angular momentum transport by thermal emission in black hole accretion disks

We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disc, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disc material. While the emission of radiation can not drive accretion on its own, we find that it does result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity-driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi-color disc is ~ 0.4r_s/r, where r_s is the Schwarzschild radius of the black hole. We briefly highlight the key similarities between this effect and the closely related Poynting-Robertson effect.

💡 Research Summary

The paper investigates how thermal radiation emitted from a viscous, multi‑color accretion disc around a black hole can transport angular momentum away from the disc. The authors begin by recalling that accretion requires the loss of specific angular momentum, traditionally attributed to turbulent viscosity, magnetic stresses, or radiative viscosity. They point out that even isotropic thermal emission carries angular momentum because photons emitted from a rotating surface experience a Doppler shift: photons emitted in the direction of motion are blueshifted, those emitted opposite are redshifted. Consequently, each photon carries a small amount of the disc’s orbital angular momentum.

Using a standard Newtonian Keplerian disc model, the temperature profile is taken as (T(r)=\bigl