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During the epoch of reionization the 21cm signal is sensitive to thescattering rate of the ultraviolet photons, redshifting across the Lyman_alpharesonance. Here we calculate the photon scattering rate profile for a singleultraviolet source. After taking into account previously neglected naturalbroadening of the resonance line, we find that photons approach the resonancefrequency and experience most scatterings at a significantly smaller distancefrom the source than naively expected r=(dnu/nu_0)(c/H), where dnu=nu-nu_0 isthe initial frequency offset, and the discrepancy increases as the initialfrequency offset decreases. As a consequence, the scattering rate P(r) dropsmuch faster with increasing distance than the previously assumed 1/r^2 profile.Near the source (r<1Mpc comoving), the scattering rate of photons that redshiftinto the Ly_alpha resonance converges to P(r) \propto r^{-7/3}. The scatteringrate of Ly_alpha photons produced by splitting of photons that redshift into ahigher resonance (Ly_gamma, Ly_delta, etc.) is only weakly affected by theradiative transfer, while the sum of scattering rates of Ly_alpha photonsproduced from all higher resonances also converges to P(r) \propto r^{-7/3}near the source. At 15<z<35, on scales of ~0.01-20Mpc/h (comoving), the totalscattering rate of Ly_alpha photons from all Lyman resonances is found to behigher by a factor of ~1+0.3[(1+z)/20]^{2/3} than obtained without fullradiative transfer. Consequently, during the early stage of reionization, thedifferential brightness of 21cm signal against the cosmic microwave backgroundis also boosted by a similar factor.

Radiative Transfer Effect on Ultraviolet Pumping of the 21cm Line in the High Redshift Universe