A Long March-4B rocket carrying X-ray space telescope to observe black holes, pulsars and gamma-ray bursts blasts off from Jiuquan Satellite Launch Center in northwest China’s Gobi Desert, June 15, 2017. (Xinhua/Zhen Zhe)
by Xinhua writers Yu Fei, Quan Xiaoshu and Qu Ting
JIUQUAN, June 15 (Xinhua) — China launched its first X-ray space telescope to observe black holes, pulsars and gamma-ray bursts, via a Long March-4B rocket from Jiuquan Satellite Launch Center in northwest China’s Gobi Desert at 11 a.m. Thursday.
The 2.5-tonne Hard X-ray Modulation Telescope (HXMT), dubbed Insight, was sent into an orbit of 550 kilometers above the earth to help scientists better understand the evolution of black holes, and the strong magnetic fields and the interiors of pulsars.
Through the telescope, scientists will also study how to use pulsars for spacecraft navigation, and search for gamma-ray bursts corresponding to gravitational waves.
The result of the wisdom and efforts of several generations of Chinese scientists, Insight is expected to push forward the development of space astronomy and improve space X-ray detection technology in China.
OBSERVATORY IN SPACE
Insight can be regarded as a small observatory in space, as it carries a trio of detectors — the high energy X-ray telescope (HE), the medium energy X-ray telescope (ME) and the low energy X-ray telescope (LE) — that cover a broad energy band from 1 keV to 250 keV, said Lu Fangjun, chief designer of the payload.
Based on the demodulation technique first proposed by Li Tipei, an academician of the Chinese Academy of Sciences (CAS), in 1993, the HE has a total detection area of more than 5,000 square centimeters, the world’s largest in its energy band.
“Given it has a larger detection area than other X-ray probes, HXMT can identify more features of known sources,” said Xiong Shaolin, a scientist at the Institute of High Energy Physics of the CAS.
Chen Yong, chief designer of the LE, said X-rays of lower energy usually have more photons, so a telescope based on a focusing technique is not suitable for observing very bright objects emitting soft X-rays, as too many photons at a time will result in over-exposure.
But HXMT won’t have that problem, as its collimators diffuse photons instead of focusing them. “No matter how bright the sources are, our telescope won’t be blinded,” said Chen.
According to Zhang Shuangnan, HXMT lead scientist, the satellite’s developers found that a set of HXMT high-energy detectors, originally designed to shield background noises caused by unwanted X-ray photons, especially those from behind the telescope, could be adjusted to observe gamma-ray bursts.
The creative new function pushes the satellite’s observation band up to 3 MeV and will get a very good energy spectrum, Zhang said.
BLACK HOLES, PULSARS
“We are looking forward to discovering new activities of black holes and studying the state of neutron stars under extreme gravity and density conditions, and physical laws under extreme magnetic fields. These studies are expected to bring new breakthroughs in physics,” said Zhang.
Compared with X-ray astronomical satellites of other countries, HXMT has a larger detection area, broader energy range and wider field of view. These give it advantages in observing black holes and neutron stars emitting bright X-rays, and it can more efficiently scan the galaxy, Zhang said.
Other satellites have conducted sky surveys and found many celestial sources of X-rays. However, the sources are often variable, and occasional intense flares can be missed in just one or two surveys, according to Zhang.
New surveys can discover either new X-ray sources or new activities in known sources. So HXMT will repeatedly scan the Milky Way for active and variable celestial bodies emitting X-rays.
“There are so many black holes and neutron stars in the universe, but we don’t have a thorough understanding of any of them. So we need new satellites to observe more,” Zhang said.
Black holes remain a mystery. One of their many secrets is why they get “angry.”
“Black holes will be the focus of our observation since they are very interesting, and can generate various types of radiation, including X-rays and high energy cosmic rays, as well as strong jets,” said Zhang.
So far about 20 black holes have been found in our galaxy. “We hope our telescope can discover more black holes. We also hope to better observe the black holes already discovered.”
If a black hole does nothing, it cannot be found. But if matter falls into a black hole, it is accelerated and heated during the process, emitting X-rays. Scientists might learn more about the characteristics of black holes from the X-rays.
Some times a black hole is calm, but other times it’s very “bad tempered.” When a black hole gets “angry,” it generates very strong X-rays or gamma ray bursts or jet-flows, Zhang explained.
Other countries have sent several X-ray satellites into orbit, but most are suitable for observing only relatively calm black holes. However, HXMT is suitable for observing angry black holes and neutron stars.
“We are still not clear why some black holes suddenly get angry, since we haven’t observed them for long enough,” he said. “We plan to make a thorough survey of black holes and neutron stars in the galaxy.”
A neutron star, or a pulsar, is so strange that when the first one was discovered, it was mistaken for signals from aliens. There are still many mysteries about this kind of star.
“We are still not clear about the interiors of pulsars. Current physical laws cannot describe the substances in the state of a pulsar well, since no lab on Earth can create a density as high as a pulsar. So we have to conduct more observations of pulsars,” Zhang said.
With their super strong gravitational and electromagnetic fields and high density, pulsars are regarded as natural laboratories of extreme physical conditions. Scientists could study many phenomena that they cannot replicate on Earth by observing neutron stars.
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 Insight.
Xiong said 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 gravitational wave events, it would increase the reliability of the detection. Combined analysis of gravitational wave and electromagnetic signals will help reveal more about the celestial bodies emitting gravitational waves, said Xiong.
Some scientists suspect that mysterious gamma-ray bursts could be electromagnetic signals corresponding to gravitational waves.
“Since gravitational waves were detected, the study of gamma-ray bursts has become more important. In astrophysics, 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 said.
HXMT’s effective detection area for monitoring gamma-ray bursts is 10 times that of the US Fermi space telescope. Scientists estimate that HXMT could detect almost 200 gamma-ray burst events a year.
“HXMT can play a vital role in searching for electromagnetic signals corresponding to gravitational waves,” said Zhang. “If HXMT can detect electromagnetic signals corresponding to gravitational waves, it would be its most wonderful scientific finding.”
“Our telescope may discover new phenomena, or even new celestial bodies. We are looking forward to new findings that nobody can predict. I hope my predictions are wrong since the most interesting astronomical discoveries are all out of expectations.”
CHINA’S SPACE SCIENCE
CAS academician Gu Yidong said China still lags behind advanced levels in space science. “We should have a sense of urgency. It will take efforts to upgrade China’s space science to advanced levels within two decades.”
Arvind Parmar, head of the Scientific Support Office in the Science Directorate of European Space Agency (ESA), said HXMT will study X-rays from objects such as black holes, neutron stars and the remains of exploded stars. These are exciting topics for scientists all over the world. HMXT will join X-ray satellites already in operation. Each mission has its own strengths.
He said the ESA has a long history of collaborating with China on scientific missions. Once HXMT is launched and starts making observations, there is great potential for joint investigations with some ESA missions. Many scientific investigations benefit from data from more than one satellite.
“I am really impressed with how China is developing its scientific space program. The recent launches of the Dark Matter Particle Explorer and the Quantum Experiments at Space Scale missions highlight China’s capabilities and commitment to science as does the range of missions under study for future launch opportunities,” said Parmar.
Paolo Giommi, a senior scientist at the Italian Space Agency, said China’s space science program foresees several satellites of increasing complexity and competitiveness. Together with the construction of large ground-based facilities, this will make China one of the major definitive producers of knowledge in space science.