We investigate a novel mechanism of bulk acceleration of relativistic outÑows of pure electronpositron pairs both analytically and numerically. The steady and spherically symmetric Ñow is assumed to start from a Wien equilibrium state between pure pairs and photons in a compact region that is optically thick to electron scattering at a relativistic temperature. Inside the photosphere where the optical thickness becomes unity, pairs and photons behave as a single fuid and are thermally accelerated. Outside the photosphere, pairs and photons behave separately, and we assume the free-streaming approximation for photons, which are emitted from the relativistically moving photosphere. Pairs are shown to be thermally accelerated further even outside the photosphere because the photospheric temperature is at least mildly relativistic. It is to be noted that the mean energy of photons is higher than that of pairs in the comoving frame of pairs and that Compton interaction leads to additional heating and radiative acceleration of pairs. For a reasonable range of the boundary temperature and optical thickness, the terminal Lorentz factor of pair outÑows turns out be more than 10 and the terminal kinetic power accounts for more than of the total luminosity. While the total luminosity should be at 2 3 least larger than the Eddington luminosity, the real luminosity can be modest if the outÑow is collimated by some unknown mechanism. This mechanism successfully avoids the difficulties of pair annihilation and radiation drag owing to pair production by accompanying high-energy photons and the strong beaming of the radiation Ðeld. It is seen that most pairs injected at the boundary survive to inÐnity. The radiation from the photosphere should be observed as MeV peaked emission at inÐnity with an order of kinetic power of jets.

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We study steady and spherically symmetric outflows of pure electron-positron pair plasma as a possible acceleration mechanism of relativistic jets up to the bulk Lorentz factor of greater than 10. These outflows are initiated by the ``Wien fireball'', which is optically thick to Compton scattering but thin to absorption and in a Wien equilibrium state between pairs and photons at a relativistic temperature.Comment: 16 pages, 13 figures, 1 table, ApJ in pres