National Aeronautics and Space Administration

The Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a matched pair of satellites developed by the Chinese Academy of Science (CAS). The two satellites work in tandem to detect gamma-ray counterparts to gravitational wave sources (such as neutron star binary system mergers): each half of the pair is able to monitor an entire hemisphere of sky from low-earth orbit, and they are located on opposite sides of the Earth in order to get a simultaneous view of the entire sky.

The two satellites were co-launched on December 10, 2020 from the Xichang Satellite Launch Center aboard a Long March 11 rocket. They were placed in 600 km altitude low earth 29° inclination orbits located on opposite sides of the Earth. Each satellite weighs about 160 kg and has an identical payload of gamma-ray detectors and charge particle detectors. The two are dubbed GECAM A (KX 08A; Xiaji) and GECAM B (KX 08B; Xiamu). The primary mission goal is to find and monitor gamma radiation from gravitational wave event sources, but the mission will also yield insights on ultra-long gamma ray bursts (GRBs), X-ray flash sources, and a variety of other transient high-energy events.

Each of the two GECAM payload includes:

• A dome-shaped array of 25 Gamma Ray Detectors (GRDs) configured to collectively see 50% of the sky. Each GRD consists of a cylindrical LaBr₃:Ce scintillator crystal which is 76.2 mm in diameter and 15 mm thick, packed in an aluminium housing to protect the crystal from moisture and stray light. The entrance window is a 0.22 mm thick Be sheet with an ESR reflector. A quartz window under the crystal provides the view for the 50.44 x 50.44 mm² SiPM 8×8 pixel array. The electronics can record in both high and low gain modes. The low gain mode provides detection in the range 5–500 keV; the high gain mode provides 30 keV–3 MeV. In the high gain mode, energy resolution was measured with laboratory sources to be 65.5% (∼3.86 keV(?)) at 5.9 keV in low gain mode, and 5.3% (∼35 keV(?)) at 662 keV. The GRD should be able to detect signals of 2.0×10⁸ ergs/cm²/s with a localization accuracy of roughly 1° for a medium-bright GRB source.
• A set of eight charge particle detectors
• Supporting space flight electronics and power supply systems, including solar panels

The satellites do not have high voltage systems and should not need to be cycled off during South Atlantic Anomaly transits.

The GECAM science objectives include:

• Localize gravitational wave counterparts detected at high energies;
• Detect and localize a variety of other transient high-energy sources: ultra-long GRBs, X-ray Flashs, Fast Radio Burst sources, and magnetars;
• Provide near-real time results to ground and other observations for rapid follow-up observations.