X-ray quadrupole emission in the scattering of two photons by a multicharged atomic ion

Pronounced spectrally extended quadrupole X-ray emission has been theoretically predicted in resonant inelastic scattering of two photons near the ionization threshold energy of the 1$s$ shell of a free multielectron atom. The observed double differential scattering cross section is estimated quantitatively. The results predicted for the Ar$^{8+}$ ion possibly concern the problem of identification of X-ray emission spectra near 3.5 keV from galaxies and clusters of galaxies.