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ISS-TAO is a mission selected for a concept study by NASA, and proposed by GSFC for launch to the International Space Station (ISS) in order to observe transient high-energy astrophysical sources. It is composed of an X-ray Wide-Field Imager (WFI), and a multi-directional Gamma-ray Transient Monitor (GTM). WFI will be built by NASA/GSFC while the secondary GTM, described in this article is contributed by the Israel Space Agency (ISA) and developed at the Technion, Israel Institute of Technology, in collaboration with Israel space industries. ISS-TAO's main science goal is to detect electromagnetic (EM) counterparts to gravitational waves (GW) detected by GW observatories, such as the Laser Interferometer GW Observatory (LIGO). Observations of simultaneous GW and EM counterparts will address fundamental questions on the nature of coalescing neutron stars and black holes as astrophysical GW sources. An EM detection will also increase LIGO’s sensitivity to detecting these events above the GW background. Promising candidates for LIGO GW sources and EM counterparts are coalescing neutron star binaries, which are now known to also emit a short Gamma-Ray Burst (sGRB). The GTM will measure these GRBs and other transient gamma-ray events, and will trigger the WFI, with or without a GW trigger. The concept of the GTM detector consists of a compact configuration of 4 segments, which will allow a fair angular resolution of a few hundred square degrees, which will facilitate a prompt follow up. Each of the GTM segments consists of a crystal scintillator, a photo-multiplier tube (PMT), followed by analog and digital electronics designed to reconstruct the energy of each incoming photon, and to yield the light-curve and spectrum of any gamma-ray transient. A central CPU then calculates the ratio of the signal of each one of the segments, and deduced the transient position relative to the GTM.
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