  Symplectic integrators are widely used for long-term integration of
conservative astrophysical problems due to their ability to preserve the
constants of motion; however, they cannot in general be applied in the presence
of nonconservative interactions. In this Letter, we develop the "slimplectic"
integrator, a new type of numerical integrator that shares many of the benefits
of traditional symplectic integrators yet is applicable to general
nonconservative systems. We utilize a fixed time-step variational integrator
formalism applied to the principle of stationary nonconservative action
developed in Galley, 2013; Galley, Tsang & Stein, 2014. As a result, the
generalized momenta and energy (Noether current) evolutions are well-tracked.
We discuss several example systems, including damped harmonic oscillators,
Poynting-Robertson drag, and gravitational radiation reaction, by utilizing our
new publicly available code to demonstrate the slimplectic integrator
algorithm.
  Slimplectic integrators are well-suited for integrations of systems where
nonconservative effects play an important role in the long-term dynamical
evolution. As such they are particularly appropriate for cosmological or
celestial N-body dynamics problems where nonconservative interactions, e.g. gas
interactions or dissipative tides, can play an important role.
