The gamma-ray transient monitor for ISS-lobster

ISS-Lobster is a mission proposed by NASA/GSFC for the International Space Station (ISS) 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 payload, described in this article is in development at the Technion, and in collaboration with Israel Aerospace Industries (IAI). ISS-Lobster's main science goal is to detect electromagnetic (EM) signals that are counterpart 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 in general relativity regarding the nature of astrophysical GW sources. An EM detection will also increase LIGO's sensitivity to detecting these events above the background. Promising candidates for LIGO GW sources are coalescing neutron stars, which are expected 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. Each of the two GTM detectors consists of a crystal scintillator, a photo-multiplier (PMT), followed by analog and digital electronics designed to reconstruct the energy of each incoming photon, and yield the light-curve and spectrum of the GRB. Analog processing reshapes the incoming pulse from the PMT, which is then converted to digital information. A Field Programmable Gated Array (FPGA) processing unit detects the pulse height and computes its photon energy. It also triggers on a significant increase in photon count rate. An embedded microprocessor on the FPGA analyzes the light-curve and spectrum over the entire GRB. This microprocessor also monitors and controls the detector, performs housekeeping tasks, and maintains communication with ISS-Lobster.