  We investigate inflationary particle production associated with a spectator
ultralight scalar field, which has been recently proposed as a plausible dark
matter candidate. In this framework, we select the Starobinsky potential to
drive the inflationary epoch, also discussing the case of a nonminimally
coupled inflaton field fueled by a quartic symmetry-breaking potential. We
focus on particle production arising from spacetime perturbations, which are
induced by inflaton fluctuations during the quasi-de Sitter stage of inflation.
In particular, we construct the first order Lagrangian describing interaction
between inhomogeneities and the spectator field, quantifying superhorizon
particle production during slow-roll. We then compare this mechanism with
gravitational particle production associated with an instantaneous transition
from inflation to the radiation dominated era. We show that the amount of
particles obtained from perturbations is typically non-negligible and it is
significantly enhanced on super-Hubble scales by the nonadiabatic inflationary
expansion. Possible implications for primordial entanglement generation are
also debated.
