Pair winds in Schwarzschild space-time with application to hot bare strange stars

Abstract

We consider a time dependent, spherically outflowing wind, in Schwarzschild space-time, consisting of electron-positron pairs and photons . Without assuming thermal equilibrium, we account for the microphysics, including two-body processes and their radiative three-body variants. We present a finite-difference scheme for solving the general relativistic kinetic Boltzmann equations for electron-positron pairs and photons. We apply this to the concrete example of a wind from a hot, bare, strange star, predicted to be a powerful source of hard X-ray photons and pairs created by the Coulomb barrier at the quark surface. We study the kinetics of the wind particles and the emerging emission in photons and pairs for stationary winds with total luminosities in the range 10^{34}-10^{39} ergs/s, for different values of the injected photon-to-pair ratio. The wind parameters--such as the mean optical depth for photons, the rates of particle number and energy outflows, bulk velocity, and number density of the pair plasma--are presented as functions of the distance from the stellar surface, as well as characteristics of the emergent radiation. We find that photons dominate in the emerging emission, and the emerging photon spectrum is rather hard and differs substantially from the thermal spectrum expected from a neutron star with the same luminosity. This might help distinguish the putative bare strange stars from neutron stars.