China has launched a pair of satellites which will watch for the electromagnetic counterparts that are emitted along with gravitational waves. These signals will be relayed quickly to the ground, helping narrow the area of sky scientists need to search to find the sources of gravitational waves.
These two 140-kilogram satellites make up the Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM), which will watch the entire sky at all times for signals from some of the most violent events in the universe 鈥� collisions between a black hole and neutron star, or mergers between two neutron stars.
The pair lifted off together from Xichang, China late on 9 December on a Long March 11 rocket.
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The satellites carry detectors for gamma rays and charged particles, and are optimised for detecting highly energetic gamma ray bursts (GRBs). The detectors will help scientists locate the source of the signal.
Most of the satellites that track events which may coincide with gravitational wave signals are in low Earth orbit. This means the planet blocks a large portion of the sky at any time. But the GECAM satellites will orbit on opposite sides of the Earth. One or both could potentially spot emissions accompanying gravitational waves.
鈥淭he crucial advantage of GECAM is that it does provide almost all-sky coverage at the same time, while not sacrificing sensitivity,鈥� says Andrew Levan at Radboud University in the Netherlands.
The mission has moved fast. Zhang Shuangnan at the Institute of High Energy Physics of the Chinese Academy of Sciences says GECAM was proposed in the wake of the first gravitational wave detection by in 2016, which was made with the Laser Interferometer Gravitational-Wave Observatory (LIGO).
The 2017 discovery of high-energy visible light counterparts for another LIGO detection 鈥� a neutron star merger event called GW170817 鈥� prompted the decision to develop and launch the mission as soon as possible. Notably the GECAM satellites are also reaching space just as LIGO and its counterpart, the Virgo interferometer, reach full sensitivity.
Peter Jonker at the Netherlands Institute for Space Research says GECAM will help astronomers better pin down the sources of gravitational waves. 鈥淭he combination of instantaneous nearly full-sky coverage with good sensitivity over a wide range of photon energies and still reasonable localisation accuracy will certainly help finding the electromagnetic counterparts to gravitational wave events,鈥� he says.
These observations can reveal the host galaxy and therefore the stellar environment that produced a gravitational wave. These tremendously powerful events can be bright but fade quickly, so the GECAM system will use China鈥檚 Beidou satellite navigation system to relay alerts rather than waiting for the satellites to pass over ground stations.
鈥淕ECAM is an acute sky watcher for sharp hunters, who will eventually catch those monsters in the sky,鈥� says Zhang.
It鈥檚 early days so we still do not know what fraction of gravitational wave sources will emit detectable gamma-rays, Levan notes. However GECAM will also enable a wealth of other great science. Zhang says GECAM is also expected to detect hundreds of gamma-ray bursts each year, solar flares and gamma-ray flashes in the Earth鈥檚 atmosphere.
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