NONEQUILIBRIUM EFFECTS IN STEADY RELATIVISTIC E(-)GAMMA WINDS()E()

We consider an ultrarelativistic wind consisting of electron-positron pairs and photons with the principal goal of finding the asymptotic Lo rentz factor gamma(infinity) for zero baryon number. The wind is assum ed 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 corr esponding to the electron mass m(e), after which non-equilibrium effec ts become important. The pairs, which carry only a small fraction of t he total energy, may be accelerated by the photons until tau falls bel ow similar to 2 x 10(-5)gamma(i)(3/4). Radiative transfer calculations show that only at this point do the radiation flux and pressure start to deviate significantly from their blackbody values. The acceleratio n of the pairs increases gamma by a factor similar to 45 compared with its value at the photosphere; it is shown to approach y(infinity) sim ilar to 1.4 x 10(3)(r(i)/10(6) cm)(1/4)gamma(i)(3/4)T(i)/m(e). The lim it of zero baryon number is a good approximation when the mass injecti on rate M (over dot) in the flow is below a critical value correspondi ng to (E (over dot)/M (over dot))(c,0) similar to 5 X 10(7)(r(i)/10(6) cm)T-i/m(e) for fixed energy injection rate E (over dot). For large ba ryon loading, E (over dot)/M (over dot) less than or similar to (E (ov er dot)/M (over dot))(c,M) similar to 350(r(i)/10(6) cm)(1/4)gamma(i)( 3/4)T(i)/m(e), the asymptotic Lorentz factor is y(infinity) similar to E (over dot)/M (over dot). Surprisingly, increasing E (over dot)/M (o ver dot) from (E (over dot)/M (over dot))(c,M) to infinity only increa ses gamma(infinity) by a factor similar to(m(p)/m(e))(1/4) approximate to 6.5, less than an order of magnitude. As E (over dot)/M (over dot) increases, the fraction of the energy carried by pairs decreases, rea ching similar to 10(-5)gamma(i)(3/4) as E (over dot)/M (over dot) --> infinity.