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/$\psi$DM) and self-interacting dark matter (SIDM) as alternative frameworks to address small-scale discrepancies in halo profiles inferred from observations. This study presents a non-parametric 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 (NFW) profile, both SIDM and $\psi$DM ($m_{\psi }=1.32_{-0.31}^{+0.22}\times {10}^{-22}\mathrm{e}\mathrm{V}$) provide significantly better fits to the resulting density profile. Moreover, the SIDM model is favored over $\psi$DM with a Bayes factor of 14.44. The reconstructed density profile features a characteristic kiloparsec-scale core ($r_c\approx 0.5\mathrm{k}\mathrm{p}\mathrm{c}$) with central density ${\rho }_c\approx 2.5\times {10}^7{\mathrm{M}}_{\odot }\mathrm{k}\mathrm{p}{\mathrm{c}}^{-3}$, 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 $\mathrm{\Lambda }$CDM substructure predictions.