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TAP, the Transient Astrophysics Probe, was selected by NASA for a Astrophysics Probe Mission Concept Study in 2017. TAP is an observatory designed to greatly advance our astrophysical understanding of the transient Universe. TAP is multi-instrument platform, sensitive to a broad variety of astrophysical phenomena across the entire sky, with rapid transient follow up with high sensitivity over a broad energy range. The baseline complementary instrument suite comprises wide-field X-ray and gamma-ray monitors, a high-resolution sensitive X-ray telescope, and a wide-field infrared telescope. This combination will discover transients deep into the Universe, and enable astrophysical characterization through broadband observations. TAP science is directly responsive to the goals set forth by the Astro2010 Decadal Survey and the NASA Astrophysics Roadmap in the areas of time domain astrophysics and gravitational waves.

TAP will address a multitude of transient astrophysical phenomena such as compact objects (black holes and neutron stars; BHs and NSs) in a large range of environments, cosmic explosions (Gamma-ray Bursts; GRBs, Supernovae; SNe), and the launch and acceleration of matter in jets (Active Galactic Nuclei, AGN; Tidal Disruption Events, TDEs). The most exciting avenue of investigation in the TAP discovery space will be the astrophysical identification of gravitational wave (GW) signals. The recent announcement of a binary BH merger detected by LIGO has generated enormous interest in the possibility of observing electromagnetic (EM) counterparts to GW sources. TAP will host a set of X-ray and near-IR instruments that will provide an optimal means for EM follow-up and localization of GW detections by the ground- based LIGO and Virgo observatories as well as the planned space-based GW observatory LISA (assuming launch dates for TAP and LISA are both in the late 2020s/early 2030s). Counterparts to very massive GW sources identified by Pulsar Timing Arrays (PTAs) may also be detectable. TAP will follow-up all GW events expected from multiple facilities across the GW frequency spectrum, spanning the BH range from a few to billions of solar masses.