Since the detection of gravitational waves, scientists have been eager to find electromagnetic signals corresponding to the gravitational waves. This will be an important task for China's space telescope, the Hard X-ray Modulation Telescope (HXMT), to be launched soon.
Gravitational waves are "ripples" in the fabric of space-time caused by some of the most violent and energetic processes in the universe. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity.
Einstein's mathematics showed that massive accelerating objects, such as neutron stars or black holes orbiting each other, would disrupt space-time in such a way that "waves" of distorted space would radiate from the source, like ripples away from a stone thrown into a pond.
These ripples would travel at the speed of light through the universe, carrying with them information about their origins, as well as invaluable clues to the nature of gravity itself.
The strongest gravitational waves are produced by events such as colliding black holes, supernovae explosions, coalescing neutron stars or white dwarf stars, the slightly wobbly rotation of neutron stars that are not perfect spheres, and the remnants of gravitational radiation created by the birth of the universe itself.
On Feb. 11, 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States announced the first observation of gravitational waves. Because these waves were generated from a black hole merger, it was the first ever direct detection of a binary black hole merger. On June 15, 2016, the second detection of a gravitational wave event from colliding black holes was announced.
Xiong Shaolin, a scientist at the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS), says the position accuracy of all the gravitational wave events detected so far is still very poor.
If scientists can find electromagnetic signals happening at similar positions and times of the gravitational wave events, it will increase the reliability of the detection. Combined analysis of the gravitational wave and electromagnetic signals will help reveal more about the celestial bodies emitting the gravitational waves, says Xiong.
Scientists have yet to detect electromagnetic signals corresponding to gravitational waves.
Many scientists would regard detecting gravitational waves and corresponding electromagnetic signals as a major scientific discovery. Some suspect that mysterious gamma-ray bursts could be electromagnetic signals corresponding to gravitational waves.
Gamma-ray bursts are extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from several milliseconds to more than an hour.
The intense radiation of most observed gamma-ray bursts is believed to be released by a supernova as a rapidly rotating, high-mass star collapses to form a neutron star or black hole. A subclass of bursts appears to originate from a different process: the merger of binary neutron stars, or the merger of a neutron star and a black hole.
About 0.4 seconds after the first gravitational event was detected on Sept. 14, 2015, NASA's Fermi Gamma-Ray Space Telescope detected a relatively weak gamma-ray burst, which lasted about one second.
But scientists disagree on whether these two events are related, and no other gamma-ray burst probe detected a gamma-ray burst. Scientists need more evidence to clarify the relationship between gamma-ray bursts and gravitational waves.
"We are not clear about many details of gamma-ray bursts. For instance, how is the energy released during a gamma-ray burst?" says Zhang Shuangnan, leadscientist of HXMT and director of the Key Laboratory of Particle Astrophysics of CAS.
"Since gravitational waves were detected, the study of gamma-ray bursts has become more important. In astrophysics research, it's insufficient to study just the gravitational wave signals. We need to use the corresponding electromagnetic signals, which are more familiar to astronomers, to facilitate the research on gravitational waves," Zhang says.
HXMT's effective detection area for monitoring gamma-ray bursts is 10 times that of the Fermi space telescope. Scientists estimate that Insight could detect almost 200 gamma-ray burst events every year. "HXMT can play a vital role in searching for electromagnetic signals corresponding to gravitational waves," says Zhang.
"If HXMT can detect the electromagnetic signals corresponding to gravitational waves, it would be its most wonderful scientific finding."
However, Zhang adds, if it cannot detect any gamma-ray bursts related to gravitational waves, it means the model suggesting gravitational waves can generate gamma-ray bursts is wrong.
Xiong says all the gravitational waves detected by LIGO were caused by mergers of black holes, which many scientists believe cannot generate electromagnetic signals. After the sensitivity of LIGO is improved in 2020, it is expected to be able to detect the gravitational waves caused by mergers of two neutron stars, which could possibly generate gamma-ray bursts.
Unlike counterparts from other nations, HXMT has unique capabilities to detect gamma-ray bursts, Zhang says. It has the largest detection area and high sensitivity in the energy range from 200 keV to several MeV.