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Session 73 - White Dwarfs, Neutron Stars and Pulsars.
Display session, Thursday, June 13
Great Hall,
In many astrophysical systems that contain accreting compact objects, the spectrum is formed predominantly by electron scattering. In this process, photons lose energy to electron recoil and gain energy from electron thermal and bulk motions. We have derived the time-dependent photon kinetic equation in both the inertial and fluid frames in three dimensions for a moving medium, allowing for absorption and emission as well as induced and spontaneous scattering. Our equation is valid in both the diffusion and free-streaming limits to second order in u/c, where u is the velocity of the scattering particles. We find important corrections to almost all previously derived kinetic equations. We show that, even if the electron gas is static, one obtains a kinetic equation that is accurate to second order in u/c only if the Klein-Nishina cross section is used. We show further that photons are systematically upscattered by the bulk motion of the electrons even if the flow does not converge, contrary to previous claims. We mention some of the implications of our results for the X-ray spectra of accreting neutron stars and black holes.
