The superconducting gap is a characteristic feature of high-$T_{\mathrm{c}}$ superconductors and provides crucial information on the pairing mechanism underlying high-temperature superconductivity. Here, we employ high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge to investigate the superconducting gap in the overdoped cuprate ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ ($T_{\mathrm{c}}=107\mathrm{K})$. By analyzing antisymmetrized, temperature-dependent RIXS spectra over a range of in-plane momentum transfers, we observe a clear suppression of low-energy spectral weight below $T_{\mathrm{c}}$, indicative of superconducting-gap formation. This suppression is most pronounced at small momentum transfers $\left[\right|{\mathit{q}}_{\parallel }|\le 0.18$ r.l.u. (reciprocal lattice units)] and corresponds to a gap size of approximately $2{\mathrm{\Delta }}_0\sim 130$ meV. Comparison with theoretical calculations of the momentum-dependent charge susceptibility supports a $d$-wave symmetry of the superconducting gap, while an isotropic $s$-wave gap fails to reproduce key experimental features. These findings establish RIXS as a powerful, bulk-sensitive probe of superconducting-gap symmetry and highlight its utility for studying materials beyond the reach of surface-sensitive techniques such as ARPES and STM.

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• Superconducting gap
• d-wave
• Cuprates
• Resonant inelastic x-ray scattering