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**Session 8 - Relativistic Astrophysics.**

*Display session, Wednesday, January 07*

*Exhibit Hall, *

## [8.01] Ultra-Relativistic Winds

*O. M. Grimsrud (Cornell University), I. Wasserman (Cornell University)*
We consider an ultra-relativistic wind consisting of
electron-positron pairs and photons with the principal
goal of finding the asymptotic Lorentz factor \gamma_\infty
for zero baryon number.
The wind is assumed to originate at radius r_i where it has
a Lorentz factor \gamma_i and a temperature T_i
sufficiently high to maintain pair equilibrium.
As r increases, T decreases and becomes less than the
temperature corresponding to the electron mass m_e,
after which non-equilibrium effects become important.
Further out in the flow the optical depth \tau drops below 1,
but the pairs may still be accelerated by the photons until \tau falls
below \sim 10^-4. Radiative transfer calculations show that
only at this point do the radiation flux
and pressure start to deviate significantly from their
blackbody values.
The acceleration of the pairs increases \gamma by a factor
\sim 45 as compared to its value at the photosphere;
it is shown to approach \gamma_\infty \sim
1.4\cdot 10^3 \gamma_i^3/4 T_i/m_e.

**Program
listing for Wednesday**