China launches space telescope to search for black holes, pulsars

 

 

 

 

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,  on June 15, 2017.   Photo by Zhen Zhe

 

 

 

 

JIUQUAN  |  2017-06-15 14:47:05

China launches

space telescope

to search for black holes, pulsars

 

By Yu Fei, Quan Xiaoshu and Qu Ting  |  CHINA FEATURES

 

 

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. on Thursday of June 15.

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.

 

 

EXPECTING SURPRISES

 

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.

 

 

 

 

 

 

JIUQUAN  |  2017-06-15 15:05:51

 

Ten facts about

China’s X-ray probe

 

By Quan Xiaoshu, Yu Fei and Qu Ting

 

 

China on Thursday launched its first X-ray space telescope, the Hard X-ray Modulation Telescope (HXMT), to study X-rays from mysterious sources, such as black holes and neutron stars. Here are 10 facts about the telescope to help you understand its origins and strengths.

 

 

 What are hard X-rays?

 

We’re all familiar with routine applications of X-rays on Earth, such as body scanning, airport security and so on. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, much shorter than that of visible light, which is why we cannot see X-rays.

Scientists divide X-rays into different energy bands, resulting in different observation methods. X-rays above 20 keV are called hard, and X-rays below 10 keV soft. X-rays between 10 keV and 20 keV are comparatively less observed in astronomy.

 

 

 How do you detect hard X-rays?

 

X-rays are absorbed by the Earth’s atmosphere, so scientists have to send detectors to high altitude by balloon, sounding rocket and satellite. But with such a small wavelength, X-rays are so energetic that they tend to pass through mirrors directly instead of reflecting off them.

As a traditional optical telescope won’t work for X-ray observation, Western scientists developed the coded mask technique and a special focusing technique called “grazing incidence.” Both are very complicated and costly, and the focusing technique requires extremely flat mirrors, which China cannot yet manufacture.

 

 

 What method did Chinese scientists develop?

 

Chinese scientists had to find a new way of imaging X-rays.

Li Tipei, a senior astrophysicist with the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), and his colleague Wu Mei came up with the direct demodulation method in the 1990s. It can help reconstruct the image of X-ray sources by using data from relatively simple non-imaging detectors, such as a telescope with “collimators” that collects and records X-ray photons parallel to a specified direction. In other words, it doesn’t need mirrors.

 

 

 Is HXMT only for hard X-rays?

 

It should be noted that HXMT, although by name a “hard X-ray” telescope, was expanded to medium and low band with two sets of detectors around 2006. Now it covers a broad energy band from 1 keV to 250 keV.

Li Tipei first proposed making a space telescope based on his demodulation technique in 1993. “At that time, it was a pioneering idea. But for many reasons, it failed to get official approval until 2011, and X-ray astrophysics had moved far ahead in the interim. So we added new scientific instruments to the satellite,” said Zhang Shuangnan, lead scientist of HXMT and director of the Key Laboratory of Particle Astrophysics at the CAS.

 

 

 What’s the structure of HXMT?

 

The 2.5-tonne telescope carries a trio of detectors, with the high energy X-ray telescope (HE) covering an energy band from 20 keV to 250 keV, the medium energy X-ray telescope (ME) from 5 keV to 30 keV and the low energy X-ray telescope (LE) from 1 keV to 15 keV.

Though different in many aspects, the three types of telescopes are basically composed of collimators, detectors and readout electronics. Collimators help shield the photons outside the field of view, detectors generate signals containing the energy and arrival information of each photon they “catch,” and readout electronics convert the signals into digital ones that can be recorded and stored.

 

 

 Why the larger detection area, the better?

 

HE has a total detection area of more than 5,000 square centimeters, the world’s largest in its energy band, ME 952 square centimeters and LE 384 square centimeters.

The larger the detection area, the more photons and signals the telescope will collect. For example, it’s easier for scientists to draw patterns out of signal curves from a detector that can “see” 100 photons per second than one that can “see” only 10 photons per second. “Given it has a larger detection area than other X-ray probes, HXMT can find more features of known sources,” said Xiong Shaolin, a scientist with IHEP.

 

 

 Why the broader field of view, the better?

 

The special focusing technique developed for X-ray observation enhances the detection sensitivity of mirror telescopes, but usually results in a narrow field of view. HXMT, on the other hand, has a very broad field of view and can finish scanning the galactic plane in about two days.

“It’s easier to spot transient sources by scanning across large sky areas, which might bring new scientific discoveries,” Xiong said.

 

 

 Is HXMT averse to very bright sources?

 

X-rays of lower energy usually have more photons. Therefore a telescope based on the focusing technique is not fit 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 Yong, chief designer of LE.

 

 

 How will an X-ray telescope detect gamma-ray bursts?

 

According to Zhang Shuangnan, scientists found that a set of HXMT high-energy detectors, originally to shield background noises caused by unwanted X-ray photons, especially those from behind the telescope, can be adjusted for the observation of 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.

 

 

 Why does HXMT have a sunshade?

 

Engineers from China Academy of Space Technology made a sunshade for HXMT low and medium energy telescopes when installing the payloads on the satellite, the first of its kind ever aboard a domestic probe.

“HE works at around 18 degrees Celsius, while ME and LE require very low temperatures, respectively down to 80 degrees below zero and 40 degrees below zero. Therefore, the satellite, just like a coat that can support a person to survive at both the equator and polar regions, needs a sophisticated temperature control system. Our solution is to cool ME and LE with the sunshade, to heat HE and insulate ME and LE from HE,” said Zhou Yupeng, deputy chief designer in charge of the satellite temperature control.

 

 

 

 

 

 

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