EWOCS-VI: Probing the hidden intermediate-mass population of Westerlund 1

Context . Westerlund 1 (Wd1), the most massive young star cluster in the Milky Way, is an excellent laboratory for studying star formation and early stellar evolution in a starburst-like environment. However, high extinction restricts studies of its stellar content, and a focus on high-mass stars limits our knowledge of the full spatial extent of the cluster. Aims . We characterize the near-infrared (NIR) variability of the stellar population of Wd1, filling the mass gap between massive stars traced by Gaia and very low-mass stars from previous Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) studies, to provide a more complete view of cluster membership across solar and super-solar masses. Methods . We exploited data from the VISTA Variables of the Vía Láctea survey and its extension (VVVX), using NIR point-spread-function photometry and astrometric solutions from its latest data release, namely the VIRAC2 catalogs, mainly in the K s band. Their large spatial coverage enables the study of both the central regions and outskirts of the cluster. We applied the HDBSCAN clustering algorithm in a 6D parameter space to differentiate cluster members from field contaminants, assessing robustness through Monte Carlo simulations. Variable sources along the line of sight were also identified and characterized. Results . We identify 1286 high-probability candidate members (12 ≤ J ≤ 18 mag) spanning ~1.5–20 M ⊙ , adopting both PARSEC 5 and 6 Myr isochrones ( A K s = 0.6, d = 4.23 kpc). A considerable fraction (~34%) show statistically significant flux variations. We present, for the first time, a parametric analysis of variability modes of Wd1 candidate members in the K s band, providing a membership catalog suitable for future kinematic studies.