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A new Chinese space mission will watch for gamma ray bursts from merging neutron stars.


China launches gamma ray–hunting satellites to trace sources of gravitational waves

The China National Space Administration’s Chang’e-5 mission, set to return Moon rocks to Earth next week, has grabbed headlines around the world. But China’s other space agency, the science-focused National Space Science Center (NSSC) of the Chinese Academy of Sciences (CAS), is making news of its own: Just after 4 a.m. local time today it launched its Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM) from the Xichang Satellite Launch Center in Sichuan province.

GECAM’s two small satellites—130 centimeters tall and weighing 150 kilograms—are now in identical 600-kilometer-high orbits, but on opposite sides of Earth. From these perches they will watch for the gamma ray bursts that emanate from the merger of ultradense objects, events that also generate gravitational waves, ripples in space-time. In 2017, astronomers witnessed this celestial light show, when a pair of neutron stars, dead cores leftover from supernova explosions, merged and spewed debris glowing at multiple wavelengths. A merger of a neutron star and a black hole are also thought to generate both light and gravitational waves. But whether a merger of two black holes should produce any sort of light is an open question, says Xiong Shaolin, an astrophysicist at CAS’s Institute of High Energy Physics and GECAM’s principal investigator. “Most theorists think the answer is no, but more and more people believe that in some circumstances it may produce electromagnetic emissions, including gamma ray bursts,” he says.

Working together, the two satellites can monitor the whole sky, tracing the source of a gamma ray burst to a particular location. Existing gamma ray observatories, such as NASA’s Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope, only have partial views of the sky, and are sometimes blocked by Earth, says Gemma Anderson, an astronomer at Curtin University. “GECAM has the whole sky covered,” she says. Also, Swift and Fermi are optimized to capture the longer, higher energy gamma ray bursts that hail from the collapse of massive stars. GECAM’s observational energy range extends down to 6 kiloelectronvolts, lower than Swift and Fermi, which may be an advantage spotting the “softer” gamma ray bursts associated with gravitational waves, Xiong says.

On the other hand, the two NASA missions, after locating a burst with their wide-field monitors, can zoom in with narrow-field instruments to study the burst’s afterglow in gamma rays and at other wavelengths. With only its all-sky monitor, GECAM will quickly alert other terrestrial and space-based instruments so they can observe the afterglow.

Péter Mészáros, a theoretical astrophysicist at Pennsylvania State University, University Park, says GECAM is something to look forward to. Swift and Fermi are aging. Swift was launched in 2004 for a planned 2-year mission; Fermi, in orbit since 2008, was expected to provide five to 10 years of service. Having another gamma ray burst detector in orbit is important, Mészáros says, because GECAM “should also continue operating after Swift’s eventual demise.”

GECAM came together quickly after Xiong and his colleagues spied an opportunity. They proposed GECAM in 2016, 1 month after gravitational wave detectors in the United States made their historic discovery of a black hole merger. The mission gives China an emerging role in what’s called multimessenger astronomy, which relies on gathering the complementary information carried by photons, gravitational waves, neutrinos, and cosmic rays that can be emitted simultaneously by cosmic events. In the case of gravitational waves, for example, the additional electromagnetic signals can help distinguish neutron star mergers from black hole-neutron star combinations, Anderson says.

Xiong’s team won funding under a 4 billion yuan ($610 million) 5-year plan through which NSSC is supporting four space science missions. In addition to GECAM, NSSC is developing the Einstein Probe, which will survey the sky for low-energy x-rays associated with violent cosmic phenomena; the Advanced Space-based Solar Observatory, to monitor the Sun’s magnetic field and watch for solar flares; and the Solar wind Magnetosphere Ionosphere Link Explorer, a joint CAS and European Space Agency mission to image Earth’s magnetosphere. Launch dates have not been finalized.