  We present a scalar-driven sterile neutrino production model where the
interaction with the ultralight scalar field modifies the oscillation
production of sterile neutrinos in the early universe. The model effectively
suppresses the production of sterile neutrinos at low temperatures due to the
heavy scalar mass, resulting in a colder matter power spectrum that avoids
constraints from small-scale structure observations. In this model, the
dominant dark matter relic is from sterile neutrinos, with only a small
fraction originating from the ultralight scalar. Furthermore, the model
predicts a detectable X/gamma-ray flux proportional to the cubic density of
local sterile neutrinos for a light scalar mass due to the light scalar
coupling to sterile neutrinos. This distinguishes our model from normal
decaying dark matter, which has a linear dependence on the density. In
addition, the model predicts a potential low-energy monochromatic neutrino
signal that can be detectable by future neutrino telescopes.
