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The photon mean free path in a relativistically moving medium becomes long inthe down-stream direction while short in the up-stream direction. As a result,the observed optical depth $\tau$ becomes small in the downstream directionwhile large in the upstream direction. Hence, if a relativistic spherical windblows off, the optical depth depends strongly on its speed and the anglebetween the velocity and the line-of-sight. Abramowicz et al. (1991) examinedsuch a relativistic wind, and found that the shape of the photosphere at$\tau=1$ appears convex in the non-relativistic case, but concave forrelativistic velocities. We further calculated the temperature distribution andluminosity of the photosphere both in the comoving and inertial frames. Wefound that the limb-darkening effect would strongly modified in therelativistic regime. We also found that luminosities of the photosphere becomeslarge as the wind speed increases due to the relativistic effects. In addition,the luminosity in the inertial frame is higher than that in the comoving frame.These results suggest that the observed temperature and brightness in luminousobjects may be overestimated when there are strong relativistic winds.

Observational Appearance of Relativistic, Spherically Symmetric Massive Winds