We study the prospects of the Lunar Gravitational-Wave Antenna (LGWA), a proposed deci-Hz GW detector, to observe binary black holes (BBHs) and enable multiband science with ground-based detectors. We assess the detectability of the events observed by current instruments up to the Gravitational-Wave Transient Catalog 4.0 data release, and of simulated populations consistent with the latest reconstruction by the LIGO-Virgo-KAGRA Collaboration. We find that the LGWA alone would have been able to observe more than one-third of the events detected so far, and that it could detect $\sim 90$ events merging in the ground-based band per year out to redshifts $z\sim 3–5$. Current detectors at design sensitivity and 100% duty cycle could detect thousands of BBHs per year, with one to a few hundred multiband counterparts in the LGWA. Third-generation (3G) detectors can observe most of the BBHs detected by the LGWA merging in their frequency band in the simulated mass range $7M_{\odot }\lesssim M_{\mathrm{tot}}\lesssim 600M_{\odot }$, enabling systematic joint analyses of hundreds of events. The short time to merger from the deci-Hz band to the Hz-kHz band (typically months to a year) allows for early warning, targeted follow-up, and archival searches. Multiband observations of intermediate-mass BBHs in the deci-Hz band are particularly promising. We perform an injection study for a GW231123-like system (the most massive BBH detection to date, which accumulates $\sim {10}^5$ inspiral cycles in the LGWA) and show that deci-Hz observations can measure the chirp mass even better than 3G instruments and yield good sky localization and inclination measurement, even with a single observatory. Opening the deci-Hz band would substantially improve the prospects of GW astronomy for intermediate-mass BBHs.

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• Gravitational wave detection
• Gravitational wave sources
• Astronomical black holes
• Binary stars
• Gravitational wave detectors