Natural $X$-ray Lines from the Low Scale Supersymmetry Breaking
  In the supersymmetric models with low scale supersymmetry (SUSY) breaking
where the gravitino mass is around keV, we show that the 3.5 keV $X-$ray lines
can be explained naturally through several different mechanisms: (I) A keV
scale dark gaugino plays the role of sterile neutrino in the presence of
bilinear $R-$partiy violation. Because the light dark gaugino obtains Majorana
mass only via gravity mediation, it is a decaying warm dark matter (DM)
candidate; (II) Compressed cold DM states, whose mass degeneracy is broken by
gavity mediated SUSY breaking, emit such a line via the heavier one decay into
the lighter one plus photon(s). A highly supersymmetric dark sector may readily
provide such kind of system; (III) Light axino, whose mass again is around the
gravitino mass, decays to neutrino plus gamma in the $R-$partiy violating SUSY.
Moreover, we comment on dark radiation from dark gaugino.
