ON THE MATHEMATICAL CHARACTER OF THE RELATIVISTIC TRANSFER MOMENT EQUATIONS

We consider general relativistic, frequency-dependent radiative transf er in spherical, differentially moving media. In particular, we invest igate the character of the differential operator defined by the first two moment equations in the stationary case. We prove that the moment equations form a hyperbolic system when the logarithmic velocity gradi ent is positive, provided that a reasonable condition on the Eddington factors is met. The operator, however, may become elliptic in accreti on flows and, in general, when gravity is taken into account. Finally, we show that, in an optically thick medium, one of the characteristic s becomes infinite when the now velocity equals +/- c/root 3. Both hig h-speed, stationary inflows and outflows may therefore contain regions that are 'causally' disconnected.