Cosmological prospects for multiband detection of intermediate-mass binary black holes with Taiji and ground-based detectors

Abstract

Intermediate-mass black holes (IMBHs) bridge the gap between stellar-mass and supermassive black holes, but remain challenging to detect electromagnetically. Gravitational-wave observations provide a direct means of detecting IMBHs and their mergers. We simulate the gravitational-wave signals of IMBH binaries under different population models and assess their detectability with the space-based detector Taiji alone and in a multiband network combining Taiji with third-generation ground-based detectors. Taiji performs well in detecting high-mass IMBH binaries, while ground-based detectors compensate for its reduced sensitivity to lower-mass systems. Their combination expands the accessible parameter space and improves the constraints on cosmological parameters. In particular, multiband observations improve the constraint accuracy on $H_0$ by $36.5\%$ and $31.0\%$ compared with Taiji and ET2CE alone, respectively. We further examine the dependence of parameter accuracy on the number of simulated events, finding that improvements are most pronounced for small samples and gradually saturate as the number of events increases. We conclude that multiband observations enhance the detectability of IMBH binaries and reinforce their role as probes of precision cosmology.

Figures (8)

• Figure 1: Characteristic strains of Taiji, ET, CE1, and CE2, together with that of a merging IMBH binary with component masses of $1000$--$1000\,M_\odot$ at redshift $z=1$. The triangle marks the GW frequency one day before coalescence. The characteristic strains are defined as $\sqrt{f S_{\rm n}(f)}$ for the detector sensitivity curves and $2f|h(f)|$ for the GW signal.
• Figure 2: Detection horizons of equal-mass, non-spinning binary black holes as a function of the total source-frame mass, for the Taiji, ET, CE1, and CE2 detectors.
• Figure 3: Normalized detection capability of black hole binaries in the redshift–primary mass and mass ratio–primary mass parameter spaces for different GW detector configurations. The color scale indicates the relative detection rate.
• Figure 4: Event distributions and corresponding SNRs of IMBBHs for one year of observation with $k = 10$, using $\{\mu_z, \sigma_z, \alpha, \beta\} = \{2, 1, 1, 1\}$. Results are shown for Taiji alone and for the Taiji--ET2CE multiband configuration. The color of each point corresponds to the SNR of the event.
• Figure 5: Same as Fig. \ref{['fig:2111']}, but assuming an intrinsic population model with $\{\mu_z, \sigma_z, \alpha, \beta\} = \{5, 1, 1, 1\}$.