ISS-TAO (a US-Israel Collaboration) and Transient Astrophysics Probe: Proposed NASA Gravitational Wave Counterpart Missions for the 2020s

Recent gravitational wave (GW) events detected by LIGO have stimulated great interest in the search for related electromagnetic counterparts. In this talk we present two GW counterpart missions now under study by NASA: ISS-TAO (Transient Astrophysics Observer on the ISS), and TAP (Transient Astrophysics Probe). ISS-TAO, a Mission of Opportunity recently awarded a Phase A study, is a wide-field X-ray transient detector to be deployed on the International Space Station in 2022, and includes an Israeli contribution of a gamma-ray transient monitor. ISS-TAO will observe X-ray counterparts of LIGO gravitational wave detections involving neutron stars and black holes. TAP, selected by NASA for a funded Probe Concept Study, is a wide-field high-energy transient satellite proposed for flight starting in the late 2020s. TAP will observe X-ray and IR counterparts of GWs involving neutron stars and stellar mass black holes detected by LIGO, and possibly X-ray counterparts of GWs from supermassive black holes, detected by LISA and Pulsar Timing Arrays. Both missions will also perform time-domain astrophysics, observing many events per year of X-ray transients related to compact objects, including tidal disruptions of stars, supernova shock breakouts, neutron star bursts and superbursts, and high redshift Gamma-Ray Bursts.