Evolution of X-ray luminosity for IMBH X-ray binaries with initial orbital period P_orb,i = 1.0 day and varying initial donor-star masses when γ = 1.60 (top panel; same as Figure 2) and γ = 1.80 (bottom panel; same as Figure 3). The solid triangles and solid stars denote the onset times at which IMBH X-ray binaries become detectable by LISA within distances of 1 and 10 kpc, respectively. The first solid triangle in the bottom panel marks the evolutionary curve for M_d, i = 3.0 M_⊙.