Colder Freeze-in Axinos Decaying into Photons
  We point out that 7 keV axino dark matter (DM) in the R-parity violating
(RPV) supersymmetric (SUSY) Dine-Fischler-Srednicki-Zhitnitsky model can
simultaneously reproduce the 3.5keV X-ray excess, and evade stringent
constraints from the Ly-alpha forest data. Peccei-Quinn symmetry breaking
naturally generates both axino interactions with minimal SUSY standard model
particles and RPV interactions. The RPV interaction introduces an
axino-neutrino mixing and provides axino DM as a variant of sterile neutrino
DM, whose decay into a monochromatic photon can be detected by X-ray
observations. Axinos, on the other hand, are produced by freeze-in processes of
thermal particles in addition to the Dodelson-Widrow mechanism of sterile
neutrinos. The resultant phase space distribution tends to be colder than the
Fermi-Dirac distribution. The inherent entropy production from late-time saxion
decay makes axinos even colder. The linear matter power spectrum satisfies even
the latest and strongest constraints from the Ly-alpha forest data.
