Constraints on the fireball parameters for the afterglow of GRB 151027B. Small black dots delineate the allowed phase space by the four constraints at 31 ks, and colored crosses indicate different possible solutions for seven different values of the kinetic energy. The solution with the lowest kinetic energy is marked with the red-filled octagon: E_kin =11.2 × 10⁵² erg, external density n = 5 cm⁻³, ϵ_e = 0.9, and ϵ_B = 1.5 × 10⁻⁵. The thick-lined triangles enclose the allowed parameter range if E_kin < 5 × E_γ,iso.