Saturday, 5 January 2019

electromagnetic radiation - On a Doppler-shift-induced radiative dissipation of angular momentum for the accretion disk of a black hole


A recent question explored the possibility that the accretion disk of a black hole could wind down and lose its angular momentum through radiative process, and Rob Jeffries' excellent answer there makes the case that this is somewhere between very unlikely and completely negligible, since most of the angular momentum radiated as light would be through circularly polarized photons, for which the density of angular momentum over energy is just not high enough.


However, there is another possible mechanism for radiation to carry away angular momentum: toward the inner parts of the disk, the gas is circling at relativistic speeds, which essentially means that its thermal emission should be Doppler shifted, both in amplitude and in photon energy, into a cone that faces its direction of travel, and this should tend to slow it down and therefore drop it to a lower orbit with less angular momentum.



I have two questions about this mechanism:



  • Is it real, and relevant? I.e. does it carry a significant fraction of the angular momentum transfers in that system? If so, in which regime?

  • More importantly, I would like to understand the nature of this radiated angular momentum, which needs to be encoded in the flat-spacetime, far-field regime of the emitted EM radiation. What form does this take? Is it encoded in the polarization or in the orbital angular momentum of that radiation? How does either of those mechanisms mesh with the fact that the emission at any given point is thermal, and therefore one would not expect coherence to play a role? What does that lack of coherence mean for the far-field wavefronts, and how does that mesh with any orbital angular momentum contributions? Or, if it's carried away as photon spin, where does that circular polarization come from?




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