Evolution of IMBH binaries with P_orb,i = 1 day and varying donor-star masses. Top panel: Characteristic strain vs. GW frequency diagram when γ = 1.60; bottom panel: γ = 1.80. The blue curve shows the LISA sensitivity based on a numerical calculation for a mission duration of four years. The red and green curves correspond to the sensitivity curves of TianQin (Wang et al. 2019) and Taiji (Ruan et al. 2020), respectively.