Spectral indexes of the Comptonization spectrum derived from the first eigenvalue of the space problem (8) in the presence of a strong magnetic field (filled circles) and for the unmagnetized case (filled squares). The explicit forms of α₁(τ₀) are given in Eqs. (28) and (37), respectively. We note that for a fixed optical depth τ₀ of the slab, the presence of the magnetic field, through reduction of the scattering cross-section, gives rise to softer spectra than an unmagnetized plasma.