Angle resolved photoemission spectroscopy (ARPES) provides a detailed view of the renormalized band structure and, consequently, is a key to the self-energy and the single-particle Greens function. Here we summarize the ARPES data accumulated over th
e whole Brillouin zone for the optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8-delta}$ into a parametric model of the Greens function, which we use for calculating the itinerant component of the dynamic spin susceptibility in absolute units with many-body effects taken into account. By comparison with inelastic neutron scattering (INS) data we show that the itinerant component of the spin response can account for the integral intensity of the experimental INS spectrum. Taking into account the bi-layer splitting, we explain the magnetic resonances in the acoustic (odd) and optic (even) INS channels.
