Top panel: critical compactness ℓ_γ,cr as a function of the minimum energy of the γ-ray spectrum ϵ_min for Γ = 1.6 (solid line) and Γ = 2 (dashed line). The numerically derived values for the two cases are shown with circles and triangles, respectively. Bottom panel: same as in top panel except for different photon indices, which are marked on the plot. Other parameters used are: ϵ_max = 2.3 × 10⁵ (in m_ec² units), B = 40 G, and R = 3 × 10¹⁶ cm. In both panels the grey area denotes the region where ϵ_min > ϵ_q; see text for the definition of ϵ_q.