A Two-level Radial-velocity Zero-point Calibration for LAMOST MRS with Gaia and APOGEE and a Value-added RV Catalogue

The LAMOST Medium-Resolution Survey (MRS) provides a large stellar spectroscopic data set for Galactic kinematics and time-domain radial-velocity (RV) studies. However, the multi-spectrograph, multi-exposure, multi-fiber observing strategy can imprint RV zero-point (RVZP) systematics that vary across instrumental and temporal hierarchies. We construct a two-level empirical RVZP correction to LAMOST DR12 MRS using Gaia DR3 magnitude-colour-corrected RVs and APOGEE DR17 RVs as external references: a spectrograph-exposure correction in (lmjm, planid, spid) units, followed by a fiber-time correction in (spid, fiberid, time tag) units. Within each unit, RVZPs are estimated from Gaia and APOGEE residuals using a weighted-median estimator, and are subtracted from the pipeline RVs to obtain corrected velocities. For high-S/N spectra, the scatter relative to APOGEE DR17 decreases from 1.1 km/s to 0.52 km/s after the full two-level correction, implying a 2x improvement in RV precision. We release a value-added catalogue of two-level-corrected RVs for 11,129,477 blue-arm spectra, including 8,158,271 single-exposure spectra and 2,971,206 coadded spectra, enabling consistent RV analyses across exposures, spectrographs, fibers, and time.