Comparison of the composite (i.e., blackbody plus power-law) HMXB spectral energy distribution (solid curve) and possible power-law alternatives (with indices α = 0.5, 1.5, 2.5, corresponding to dotted, short-dashed, and long-dashed curves, respectively). The composite HMXB spectrum is as measured at t = 50 Myr in Figure 1, while the power-law spectra are normalized such that the total energy emitted between 1 and 2 keV is the same for both the composite and power-law spectra.