Gradient of the spherically averaged profiles of the differential brightness temperature around the first light source appearing at z = 14.06 in the 100  h^-1 Mpc simulation. The four panels show the profile at different redshifts, from z = 13.22 to z = 11.64. At each redshift, arrows show the predicted position of the Lyϵ, Lyδ and Lyγ horizons. The Lyϵ and Lyδ horizons can be detected during a redshift interval Δz ~ 2. The fainter Lyγ horizon is visible for a shorter period Δz < 1.5.