  The putative recent indication of an unidentified 3.55 keV X-ray line in
certain astrophysical sources is taken as a motivation for an improved
theoretical computation of the cosmological abundance of 7.1 keV sterile
neutrinos. If the line is interpreted as resulting from the decay of Warm Dark
Matter, the mass and mixing angle of the sterile neutrino are known. Our
computation then permits for a determination of the lepton asymmetry that is
needed for producing the correct abundance via the Shi-Fuller mechanism, as
well as for an estimate of the non-equilibrium spectrum of the sterile
neutrinos. The latter plays a role in structure formation simulations. Results
are presented for different flavour structures of the neutrino Yukawa couplings
and for different types of pre-existing lepton asymmetries, accounting properly
for the charge neutrality of the plasma and incorporating approximately
hadronic contributions.
