Limiting A_gwb (at f = 1 yr⁻¹) that could be detected with each of the nine γ-ray concepts discussed in the main text and Table 1. Only the results from scenario S1 are shown here since S2 and S3 are all but identical. The gray shaded region gives the approximate range of candidate GWB signals currently detected by radio PTAs, ﹩{{ \mathcal A }}_{15}=2﹩–3. Performance below the gray band enters the self-noise regime from the GWB, and other features of the PTA (e.g., sky coverage, number of pulsars) become consequential.