(Judith Racusin, Maxime Rizzo, Brad Cenko, Sarah Logsdon, Jeremy Perkins, Julie McEnery, Qian Gong)
Over the last decade, the Fermi gamma-ray space telescope has provided pioneering discoveries in the field of high-energy astrophysics. This includes the first detection of a short gamma-ray burst (sGRB) coincident with a gravitational wave event from merging neutron stars using its Gamma-ray Burst Monitor (GBM) instrument. This set-off a world-wide campaign to identify and characterize the associated kilonova. Here we describe Nimble, a NASA Explorer class mission concept that couples a GBM like very wide-field monitor with a UV-Optical-IR telescope and rapid response spacecraft to enable the prompt detection and localization of sGRBs and follow-up of kilonovae. Nimble will reveal the nature of neutron star merger multi-messenger events and its flexible platform will allow for a wide range of additional time domain science.