  Magnetic excitations in the optimally doped high-$T_\mathrm{c}$
superconductor Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta}$
(OP-Bi2201, $T_\mathrm{c}\simeq 34$ K) are investigated by Cu $L_3$ edge
resonant inelastic x-ray scattering (RIXS), below and above the pseudogap
opening temperature. At both temperatures the broad spectral distribution
disperses along the (1,0) direction up to $\sim$350~meV at zone boundary,
similarly to other hole-doped cuprates. However, above $\sim$0.22 reciprocal
lattice units, we observe a concurrent intensity decrease for magnetic
excitations and quasi-elastic signals with weak temperature dependence. This
anomaly seems to indicate a coupling between magnetic, lattice and charge modes
in this compound. We also compare the magnetic excitation spectra near the
anti-nodal zone boundary in the single layer OP-Bi2201 and in the bi-layer
optimally doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_2$O$_{8+\delta}$
(OP-Bi2212, $T_\mathrm{c}\simeq96$ K). The strong similarities in the
paramagnon dispersion and in their energy at zone boundary indicate that the
strength of the super-exchange interaction and the short-range magnetic
correlation cannot be directly related to $T_\mathrm{c}$, not even within the
same family of cuprates.
