A Dense Dark Matter Core of the Subhalo in the Strong Lensing System JVAS B1938+666

The nature of dark matter remains unknown, motivating the study of fuzzy/wave dark matter (FDM/ψDM) and self-interacting dark matter (SIDM) as alternative frameworks to address small-scale discrepancies in halo profiles inferred from observations. This study presents a nonparametric reconstruction of the mass distribution of the previously found dark subhalo in the strong lensing system JVAS B1938+666. Compared with the standard Navarro–Frenk–White profile, both SIDM and ψDM ( ) provide significantly better fits to the resulting density profile. Moreover, the SIDM model is favored over ψDM with a Bayes factor of 14.44. The reconstructed density profile features a characteristic kiloparsec-scale core (rc ≈ 0.5 kpc) with central density , exhibiting remarkable consistency with the core–halo mass scaling relations observed in Local Group dwarf spheroidals. These findings offer insights that may help address the core–cusp discrepancy in ΛCDM substructure predictions.