Solid gray line: locus of source ages and distances giving the same flux as the maximal one ψ_M yielded by a source located at distance d_c from the Earth, attained for . Dashed gray line: locus of source ages and distances giving the same flux as the one of a source of age τ_c and distance d_c. In the case shown (τ_max<τ_c), the flux is also the maximal flux from a source farther than d_c and older than τ_c.