FAST (Five-hundred-meter Aperture Spherical radio Telescope)FAST Overview Mission Status References
FAST is a Chinese radio telescope. It is the world's largest and most sensitive radio telescope and three times more sensitive than the Arecibo Observatory. FAST is managed by NAOC/CAS (National Astronomical Observatories/Chinese Academy of Sciences) and funded by NDRC (National Development and Reform Commission). The ultimate goal of FAST is to discover the laws of the development of the universe. 1)
Figure 1: In this photo,released by the Xinhua News Agency on 24 Sept. 2016, an aerial view shows the Five-hundred-meter Aperture Spherical Telescope (FAST) in the remote Pingtang county in southwest China's Guizhou province (image credit: Liu Xu, Xinhua News Agency)
Measuring 500 meters in diameter, the radio telescope is nestled in a natural basin within a stunning landscape of lush green karst formations in southern Guizhou province. It took five years and $180 million to complete and surpasses that of the 300-meter Arecibo Observatory in Puerto Rico, a dish used in research on stars that led to a Nobel Prize.
Installation of the 4,450-panel structure, nicknamed Tianyan, or the Eye of Heaven, started in 2011 and was completed in July 2016. The telescope requires a radio silence within a 5 km radius, resulting in the relocation of more than 8,000 people from their homes in eight villages to make way for the facility, state media said. Reports in August said the villagers would be compensated with cash or new homes from a budget of about $269 million from a poverty relief fund and bank loans.
The FAST telescope will spend the coming decades exploring space and assisting in the hunt for extraterrestrial life. And once it commences operations in September 2016, the Chinese expect it will remain the global leader in radio astronomy for the next ten or twenty years. FAST is capable of forming a parabolic mirror. That will allow researchers a greater degree of flexibility. 2) — On September 25, 2016, the FAST telescope began operating in southwestern China.
FAST uses a data system developed at ICRAR (International Center for Radio Astronomy) in Perth, Australia and at ESO (European Southern Observatory) to manage the huge amounts of data it generates. The software is called NGAS (Next Generation Archive System), and will help astronomers using the telescope to search for rotating neutron stars and look for signs of extra-terrestrial life. The NGAS data system will help to collect, transport and store about 3 PB (Petabytes, 3 x 1015) of information a year from the telescope. 3)
The idea of sitting a large spherical dish in a karst depression is rooted in Arecibo telescope. FAST is an Arecibo-type antenna with three outstanding aspects: the karst depression used as the site, which is large to host the 500-meter telescope and deep to allow a zenith angle of 40 degrees; the active main reflector correcting for spherical aberration on the ground to achieve a full polarization and a wide band without involving complex feed systems; and the light-weight feed cabin driven by cables and servomechanism plus a parallel robot as a secondary adjustable system to move with high precision. The feasibility studies for FAST have been carried out for 14 years, supported by Chinese and world astronomical communities. Funding for FAST has been approved by the National Development and Reform Commission in July of 2007 with a capital budget ~ 700 million RMB. The project time is 5.5 years from the commencement of work in March of 2011 and the first light is expected to be in 2016. 4)
An international review and advisory conference on science and technology of FAST was held in Beijing in March 2006. The review panel unanimously concluded that the FAST Project is feasible and recommended that the project moves forward to the next phase of detailed design and construction as soon as possible. Funding for project FAST has finally been approved by the National Development and Reform Commission (NDRC) in July of 2007. The approved budget is now 700 million RMB. At the end of 2008, the foundation has been laid. The construction period is 5.5 years from the commencement of work in March 2011. The first light is expected to be in 2016.
FAST is an Arecibo-type spherical telescope. Figure 2 illustrates the optical geometry of FAST and its three outstanding features: the large karst depressions found in south Guizhou province as the sites, the active main reflector of 500 m aperture which directly corrects for spherical aberration, and the light-weight feed cabin driven by cables and a servomechanism plus a parallel robot as a secondary adjustable system to carry the most precise parts of the receivers. Inside the cabin, multi-beam and multi-band receivers will be installed, covering a frequency range of 70 MHz - 3 GHz. The telescope will be equipped with a variety of instruments and terminals for different scientific purposes.
Figure 2: Left: FAST optical geometry, right: FAST 3-D model (image credit: NAOC/CAS)
Surveying Site: A practical way to build a large spherical telescope is to make extensive use of existing depressions which are usually found in karst regions. Site surveying in Guizhou province started in 1994, including geo-morphological features and the distribution of the karst depressions, climate, engineering environment, social environment, and radio interference. At least 400 candidate depressions were investigated with remote sensing, and the Geographic Information System. The expense of earthwork largely depends on the geometrical profile of a depression.
The Dawodang depression in south Guizhou has been selected as the telescope site (Figure 3). Total earthwork is estimated to be ~ 1,000,000 m3 according to the comparison of the reflector model with the digital terrain model images of the depression. The relatively low latitude (~26°N) of the site enables the observation of more southern galactic objects. The mild climate of the subtropical zone with a few days of frost and snow without ice build-up enable survival of low cost structures. There is no inundation of karst depressions because of their good drainage, but a tunnel is still budgeted in order to ensure telescope safety. No devastating earthquake has ever been recorded in history. The remoteness and sparse population guarantee a clean RFI environment and the safety of future FAST instruments. An agreement on a temporary radio quiet zone around the site has already been signed by the Chinese Academy of Sciences (CAS) and Guizhou provincial government.
Figure 3: Depression Dawodang, East: 107º21' North: 25º48', Altitude: ~1000 m. Right: image by quick bird with resolution of 0.6 m, the dimension of the circle is ~1000 m (image credit: NAOC/CAS)
Requirement from FAST sciences: FAST sciences put stringent limits on surface deformation of the main active reflector. The position of all the nodes need to be controlled precisely, which depends not only on the structure of the cable net, the back frame and the reflector elements, but also on status of thousands of down-tied cables and actuators. The tracking of FAST is realized by adjusting the reflector in real-time. So the speed of the deformation, which is related to the speed of actuators, should be high enough (about 15° per hour in the extreme case) to track a target in the sky. The shape and size of the panels are carefully chosen to minimize the required deformation from a sphere to form a paraboloid and to reduce intrinsic polarization.
Fundamental Questions for a Large Single Dish Radio Telescope: The origin of the observable universe, the origin of our world with the Sun and the Earth, and the origin of intelligent life are overarching questions of natural sciences. FAST, with its unparalleled collecting area, state of art receiver systems, and the digital backend of which the technology development largely follows the Moore‘s law, has a unique window for contribution through precise measurements of matter and energy in the low frequency radio bands.
At radio frequencies, a large single dish telescope is capable of observing the main component of cosmic gas, atomic hydrogen (HI), from the local universe to moderate redshifts. The gaseous galaxies can be either bright or totally dark in optical bands depending on their history of star formation. Therefore, a complete census of gaseous universe through blind surveys provides information of cosmology and galaxy evolution independent of those based on star light. One exciting development of research in cosmology is the apparent success of ΛCDM (”Double Dark” Standard Cosmological Model) simulations based on models in producing large-scale structures of dark matters. This is accomplished without knowing the actual content of neither dark matter nor dark energy. The critical test of such models and associated cosmology is to compare predicted structures to observable matters. One current mystery is the so called ”missing satellite problem”, i.e., the lack of detection of low mass halos predicted by dark matter simulations. Given the uncertainties in our knowledge of star formation (discussed later) and the very rudimentary treatment of star formation in simulations, the stellar content of these halos is essentially unknown. Therefore, by providing a census of HI complete to relatively low mass limit along with other large radio telescopes, FAST can significantly improve our knowledge of the origin of the universe.
Pulsars: The high sensitivity and larger sky coverage compared with Arecibo make FAST a powerful tool for detecting pulsars at large distances, such as millisecond pulsars, binary pulsars, double pulsars, extragalactic pulsars, etc. It is estimated that FAST equipped with multi-beam receivers would detect thousands of pulsars in the Milky Way Galaxy in less than a year of observing time. In such a new large scale survey, moreover, extremely interesting and unknown exotic objects may yet wait for discovery by the sensitive FAST as the telescope is put into operation. Among these discoveries, the most exciting one should be a pulsar-black hole binary, which will provide precise information of black hole. Besides this, FAST may also find sub-millisecond pulsars and pulsars that have a mass deviating from the 1.4 solar mass. This will give insight to the equation of state at supra-nuclear density, and further the strong interaction. In this way, a pulsar is a unique laboratory for studying two of four kinds of fundamental forces: gravitation and strong interaction.
Main active reflector: As a huge scientific device, the supporting structure of the radio telescope FAST demands special requirements beyond those of conventional structures. The most prominent one is that the supporting structure should enable the surface formation of a paraboloid from a sphere in real time through active control. Fortunately, the peak deviation of the paraboloid of revolution from the spherical surface is only about 0.67m across the illuminated aperture of about 300 m. An adaptive cable-net design has been proposed for two main reasons: first, the small difference required by the deformation mentioned above should be easily achieved within the elastic limit of ordinary cable wires; second, the cable-net structure should accommodate with the complex topography of karst terrain easily, which will avoid heavy civil engineering between actuators and the ground.
Figure 4: Concept of the adaptive cable-net structure, the supporting structure for the FAST reflector (image credit: NAOC/CAS)
Leading the international VLBI (Very Long Baseline Interferometry) network: Angular resolution in astronomy is defined as the angular separation of two astronomical objects whose images can just be resolved. The angular resolution is θ=λ/D, which is the reciprocal of the aperture in units of wavelength. For a radio telescope, the wavelengths used are a million times larger than those in the optical band. If we want to obtain a resolution equivalent to the optical, we would have to make a "big dish" hundreds of kilometers in diameter, or even as large as the earth, and the surface deviations of the panels need to be kept to 1 millimeter, or even less. The necessary technology does not exist. Radio astronomers have found another way to improve the resolution without enlarging the antenna aperture — the radio interferometer. This has finally developed into the present very long baseline interferometry (VLBI). Two antennas joined in VLBI can sit on different continents, giving an angular resolution of θ=λ/B. The baseline B can be as long as the diameter of the Earth, and even longer if we send antennas into space. The resolution of a modern global VLBI network is finer than a marcsec (milliarcsecond), and is three orders higher than that at other bands. The main VLBI networks in the world are the EVN (European VLBI Network), VLBA (Very Long Baseline Array, USA) and APT (Automated Patrol Telescope, University of New South Wales, Australia). The main antenna apertures are 20-40 m, the diameter of the largest antenna is 100 m. When the 500 meter aperture telescope joins in the VLBI networks, it will naturally become the dominant dish due to its huge collecting area and a favorable location at the edge of all the existing networks. By then, China will be the leading force in international VLBI cooperation.
Detecting interstellar communication signals : SETI (Searching for Extra-Terrestrial Intelligence) is usually considered to be a high-risk task. However, if it succeeds, it will overshadow all other scientific achievements of mankind. Therefore, exploration by the scientific community, and support for SETI from governmental and non-governmental organizations in developed countries have never stopped.
The only available way for communicating with civilizations on distant planets is to search for extra-terrestrial "artificial" electromagnetic signals. The non-thermal Galactic background emission, quantum noise and cosmic microwave radiation are three noise sources which exist everywhere. Engineers in extra-terrestrial civilized societies also face a similar radio noise spectrum, and they might use the same microwave window as us.
The SETI experts believe that humans should concentrate the search in a frequency range from 1-3GHz, especially between the 21 cm neutral hydrogen HI line and the 18 cm OH line. The combination of H and OH forms water, H2O, so the narrow frequency band is also called the "water hole". Since water is the basic element for life on Earth, the extra-terrestrial "water population" would probably naturally search for us through the water hole.
The Phoenix project is one of the most ambitious SETI plans. It began in 1994, and searched for microwave signals from about 1000 nearby solar-like stars, using the biggest antenna in the world. In 2006, the ATA (Allen Telescope Array), supported by American private enterprise, began partial operation. The array will consist of 350 dishes, covering an area of 300 x 200 m2. It is specially designed for SETI science, to detect interstellar communication.
Calculations reveal that if we use an omnidirectional antenna with a transmitter power of 1000 MW (for comparison, the EIRP of a typical television station is about 1 MW, and the radiated power of the most powerful transmitter on Earth is about 10 million MW), then:
The Parkes 65 m telescope in Australia could detect the signal to 4.5 light years, and it would reach only one star — α Centauri. The Arecibo 305 m telescope detection distance is 18 light years, and it could reach 12 stars. FAST could search out to 28 light years, and would be able to reach 1400 stars. If we increase the transmitter’s radiated power to 1000,000 MW, Parkes could reach 5000 targets, while FAST would be able to reach a million stars.
FAST has three outstanding features: the unique karst depressions found in south Guizhou as the sites, the active main reflector of 500 m, which directly corrects for spherical aberration, and the low-mass focus cabin driven by cables and a servomechanism plus a parallel robot as a secondary adjustable system to carry the most precise parts of the receivers. Inside the cabin, multi-beam and multi-band receivers will be installed covering a frequency range of 70 MHz - 3 GHz. The telescope will be equipped with a variety of instruments and terminals for different scientific proposes. The main technical specifications of FAST are listed in Table 1. 5)
Table 1: FAST main technical specification
The FAST technical subsystems are (Figure 5):
• site exploration and earthwork
• observatory construction.
Active reflector system:
The FAST active reflector includes a main 500 meter aperture cable mesh composed of ~7000 strands of steel cables, reflecting elements, actuators, ground anchors, perimeter beam, wind-shield wall, noise-shield wall, etc. The reflecting element cable mesh is installed on the annular latticed perimeter beam. There are 2400 nodes in the network, by which 4600 reflecting panels are installed on the cable mesh to reflect the radio wave. Every node is connected with a down-tied driving cable and an actuator device, which is then connected with the ground anchor. A noise-shield wall is installed around the perimeter of the reflector, and outside is a wind-shield wall. All of these devices form a complete active reflector system. The construction of this system is aimed to build a 500 meter aperture active spherical reflector, which could realize to form a transient 300 meter parabolic dish under real time control.
The function of the reflector is to reflect the electromagnetic wave to the focus, so that the receiver can receive and record the signals. Also it can transform actively and hold the weight of the back frame, panels and wind load.
Feed Cabin Suspension System:
The feed cabin suspension system includes the following structure:
- The optical, mechanical and electronic integration first-order cable support system: 6 tower supports with height of about a hundred meters are built in the mountain around the depression. A kilometer-scale steel cable soft support system and the guide rope, cable reel are installed to realize the first-order spatial position adjustment of the feed cabin.
- The 10 m diameter feed cabin. A parallel robot is installed in the feed cabin for the second-order adjustment to realize a spatial position accuracy of 10 mm.
- The steering gear between first-order and second-order adjustment mechanisms to help adjust the attitude angle of the feed cabin.
- The power and signal channels between ground and the feed cabin.
- The safety and health monitor system: It includes lightning protection, cable stress force monitor, emergency prevention and dealing equipments.
Figure 6: Schematic view of the feed cabin suspension system (image credt: NAOC/CAS)
Measurement and Control System:
There is no rigid structure connecting the FAST reflector and focus. Therefore it requires high-accuracy measurements of their spatial coordinate in a common well-defined reference frame. Besides, all moving parts of the telescope require real-time measurement and reaction control during operations to meet the position accuracy requirement. Various modern measurement technology will be applied to the site exploration, earth work, reflector and feed cabin support, receiver and terminal systems, and the real-time detect and health monitor of the telescope during operation in future. Large dimension, high sampling rate, high accuracy measurement and control is the key points.
We plan to build a reference datum net comprised of 10 datum station with mm-accuracy. There are two most challenging tasks to be accomplished.
1) Real-time reaction control of the feed cabin: to read the 3D spatial positions of the feed cabin, four API laser trackers are used to measure 4 follow-up targets in the Stewart platform. Two API laser tracker systems are used to measure the position and attitude of the lower platform, and monitor the control results.
2) Surface scanning of the reflector: There are more than 2000 nodes, the joining points of the element panels on the reflector, and the number of nodes being illuminated is ~1000. As a plan, nine close-range instrumentations with accurate rotating platforms and digital cameras will be built to scan 1000 control nodes in the illuminated portion of the reflector during observations.
The project is also developing control technology to realize the spatial positioning of the receiver in the feed cabin.
The feeds and receivers are planned to be built through international cooperation, which cover a frequency range from 70 MHz to 3 GHz. The present 9 feeds and receivers in plan are displayed in Table 2. 7)
Table 2: Specification of the receivers
The proposed construction mainly includes:
(1) Feeds and low-noise receivers: to develop feeds and polarizer at 9 bands, LNAs, band selection filters, radio frequency circuit, frequency mixers and IF circuits.
(2) Refrigeration machine: Helium GM refrigeration machines and vacuum Dewar are used to refrigerate the head amplifier and other key instruments, especially for receiver at bands higher than 560 MHz, which requires a temperature of 10-20 K to keep low noise.
(3) Optical fiber to transform IF data: to develop a wide-band optical fiber transformation. IF data is converted to light signal, and then transform in the optical fiber over a distance of about 3 km, and convert back to IF data to the digital terminals.
(4) Digital data-processing terminals: to construct multi-band terminals for cosmic neutral hydrogen, pulsar de-dispersion, molecular lines observations, VLBI data-record terminal, SETI terminal and digital record and process terminal based on computer clusters.
(5) Receiver monitor and diagnosis system: to monitor the receiver in real-time, and test the remote trouble diagnosis to shorten the feed cabin maintenance time in harbor.
(6) The time frequency standard: this will be provided by GPS and high stationary hydrogen clock to fulfill the requirements of pulsar, spectral lines and VLBI observations, etc.
Mission status and some results
• July 21, 2022: A large-scale neutral hydrogen (HI) survey of the local universe is one of the major science initiatives under the Five-hundred Meter Aperture Spherical radio Telescope (FAST) project. 8)
- Equipped with a 19-beam array receiver and combined with super-high sensitivity due to its large collection area, FAST is the most powerful survey tool for exploring the HI universe.
Figure 7: Artist illustration of three types of objects that can be discovered by FAST: Fast radio burst (FRB), Pulsar and HI galaxies (image credit: Qian Lei)
- The late Prof. Nan Rendong, who founded the FAST project and served as its chief scientist and engineer, noted that the FAST HI survey would be able to address many important astrophysical questions. It could investigate the extent of HI disks, study the extended rotation curve up to unprecedented distances, map the distribution of dark matter in the local group, search for low-mass galaxies in void regions, and detect cold dark matter satellites and possible HI companions.
- The FAST HI survey team, led by Dr. Zhu Ming from the FAST Operation and Development Center at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), has been actively scanning the sky ever since FAST formally began operation in January 2020. The team made its first data release on June 19 in a paper published in Research in Astronomy and Astrophysics. 9)
Figure 8: Sky coverage of the FAST HI survey. The red area is observed using the 19-beam receiver, while the blue area is observed using an ultra-wide band receiver. (image credit: FAST HI survey team)
- "This is a pilot study to test the observational strategy and data reduction pipeline for the large-scale HI survey, and the results demonstrate that FAST indeed has a high survey speed and can obtain HI spectra with higher spectral resolution than previous surveys," said Dr. Zhu Ming, corresponding author of the paper.
- The catalog release from the pilot survey contains 544 HI-detected galaxies. The most interesting result is that 16 of the FAST-detected sources have no optical counterparts, which means that FAST can discover galaxies that are possibly missed by optical telescopes.
- Dr. Zhu said the objects are very interesting to galaxy formation theory and may represent a new type of object that contains dark matter and HI gas but has few stars because it fails to form them. "Such type of object is predicted by cosmology models but has never really been confirmed by observation," said Dr. Zhu.
Figure 9: "Dark" galaxies found by FAST but not seen by optical telescopes. Left and middle panels show HI spectrum and image observed by FAST, respectively; the right panel is an optical image (image credit: Kang Jiangang)
- To confirm whether seemingly "dark" galaxies really have no optical emissions, however, scientists will need very deep optical observations with a large telescope along with the FAST HI survey data.
- Besides dark matter, galaxies also contain gas and stars. The gas is mainly in the form of HI, which is detectable by radio telescopes through an emission spectral line at a 21-cm wavelength.
- So far, only about 35,000 gas galaxies have been detected. However, now that the FAST extragalactic HI survey is running at full speed, it has the potential to discover hundreds of thousands of gas galaxies as well as dark galaxies, tidal tail filaments, accretion streams, and ultra-diffuse dwarf galaxies.
• March 17, 2022: A correlation between the observed frequencies and polarizations of the energetic radio pulses suggests the bursts originate in active regions such as magnetars in binary systems. 10)
- Fast radio bursts, or FRBs, are very short, extremely bright extragalactic radio flashes that light up the sky thousands of times a day. The bursts peak at frequencies between 400 MHz and 8 GHz, and they sometimes but not always repeat.
- Since the first FRB signal was unearthed from archived observation data 15 years ago, scientists have tried to determine their origin. Thanks to highly sensitive radio telescopes on Earth, several candidate sources have emerged. For example, astronomers determined in 2017 that the well-studied repeating FRB 121102 dwells in a bright star-forming region inside a dwarf galaxy 3 billion light-years away. And two years ago researchers traced FRB 200428 to a distant magnetar, a neutron star with an ultrastrong magnetic field (see Physics Today, January 2021, page 15).
Figure 10: The Five-Hundred-Meter Aperture Spherical Radio Telescope in southwestern China is the largest single-dish radio telescope in the world (photo credit: Yongkun Zhang)
- In a paper published in the 18 March Science, researchers analyzed the polarization of active FRBs and offered the first systematic description of the environments in which they reside. The results indicate that the bursts likely originate in young or complex environments such as supernova remnants. 11)
- “This paper confirms that FRBs only occur in special places in the universe,” says Ue-Li Pen of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, who was not involved with the research. “It’s an important step toward better understanding the immediate environments of those mysterious bursts.”
- An FRB typically lasts for a few milliseconds but releases about as much energy as the Sun does in a year. The raw energy of the outbursts has led astronomers to suspect that extremely powerful magnetic fields, like those of a neutron star, are required to produce them. If that is the case, then the magnetic fields should also strongly polarize the outgoing burst of radio waves. Indeed, in 2018 the Green Bank Telescope in West Virginia detected nearly 100% polarization from FRB 121102. 12)
- But in 2019, when the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) detected 1652 eruptions from FRB 121102, no polarization was observed. “It struck us as a big surprise, since other telescopes had seen high polarization from the same source, and FAST has unparalleled sensitivity,” says paper coauthor Di Li of the National Astronomical Observatories of China in Beijing. He is the chief scientist of FAST, the world’s largest single-dish radio telescope, which sits in a natural depression in southwestern China.
- Li and colleagues wondered whether their surprising finding would apply to other bursts. So they compiled the polarization measurements of 21 repeating FRBs, which account for about 90% of all the known repeating bursts, for a systematic examination. They discovered that polarization increases and decreases with the observed frequency of the bursts. At low frequencies, the bursts can be completely unpolarized. “We found 2 GHz to be the threshold,” Li says. That would explain why FAST failed to detect polarization: It probed FRB 121102 at 1–1.5 GHz, whereas the Green Bank Telescope’s polarization finding came at frequencies of 3 GHz and above.
- To explain why the polarization of FRBs drops quickly at lower frequencies, the team proposed a picture in which the immediate environment—rather than the source itself—plays a central role. Although the bursts are likely produced at their source fully polarized, they travel through the nearby gas or plasma on slightly different paths on the way to telescopes on Earth. In the process, the waves encounter different electron densities and magnetic field strengths in front of or around them, which leads to slightly different Faraday rotations of their polarization angles. The magnitude of the rotation is greater at lower frequencies than at higher ones. By the time the lower-frequency waves are picked up by a telescope’s receiver, they no longer show the initial polarization and appear depolarized. “Averaging over multiple paths, the polarization in lower frequencies is more smeared than in higher frequencies,” says paper coauthor Bing Zhang of the University of Nevada, Las Vegas.
- The findings in the Science paper fit into our current understanding of repeating FRB sources as relatively young, energetic objects, and they are consistent with studies that located some of the sources in star-forming regions within galaxies, says Jason Hessels of the University of Amsterdam in the Netherlands.
- If an FRB source has ionized gas or plasma around it with magnetic fields capable of depolarizing radio waves, it means the environment is relatively young, explains Pawan Kumar of the University of Texas at Austin. A common scenario is a supernova remnant, where a newborn neutron star lives at the heart of a cloud made of leftover material from the explosion of the precursor star. It takes tens of thousands of years for the cloud to disperse, but the neutron star will remain active much longer after that, says Kumar.
- Loss of polarization can also indicate a not-so-young but relatively complex environment. For instance, some neutron stars live close to the supermassive black hole at the center of a galaxy. Radio bursts from such neutron stars would need to go through the gas or plasma that surrounds the black hole.
- Another example of a complex environment is a magnetar that orbits another star. The pair may interact and create a turbulent environment within which the magnetar’s bursts would scatter and depolarize. “The magnetar in this scenario doesn’t have to be very young,” Zhang says.
- All those scenarios may turn out to be valid. There used to be more than 50 theoretical models about the physical origin of FRBs. All but a few have been ruled out, but that doesn’t mean that only one offers a correct description. “To think that there’s only one origin would be underestimating nature’s creativity,” Zhang says.
Figure 11: The degree of linear polarization for multiple fast radio bursts (nine shown here, designated by color) increases with the burst frequency. The shapes represent the radio telescopes that made the observations. Each burst has a different rotation measure (RM), which quantifies the Faraday rotation of polarization [image credit: Y. Feng et al., Science 375, 1266 (2022)]
- If the paper’s conclusions are confirmed by future detections, it could be a major step toward completing the cosmic puzzle of FRBs. “I’m glad the team was able to model the data relatively early,” Pen says. “This is one of those papers that I look at and I’m thinking, ‘It’s a very good result, and we should’ve done it first.’”
- Pen believes the field will see major discoveries within the next five years. He is leading the development of a telescope called the Bustling Universe Radio Survey Telescope in Taiwan, which has a large field of view and long baseline tailored for observing FRBs relatively close to Earth. Once complete, the telescope will join the Canadian Hydrogen Intensity Mapping Experiment and other radio telescopes to accurately locate the host galaxies of FRBs, including one-off bursts. Researchers want to answer the question of how many types of FRBs exist: Are they mostly single events, or do they all repeat at some cadence, whether separated by only the blink of an eye or spaced out over hundreds or thousands of years?
- The FRB community also hopes to use the frequent bursts to understand the evolution of the universe. Since FRBs are the only coherent radiation sources from the deep universe, researchers have proposed using bursts that are gravitationally lensed by nearer galaxies to make measurements of features including dark matter, baryonic matter, and the expansion rate of the universe. And by detecting and pinpointing the origin of enough extremely distant bursts, astronomers may be able to use FRBs to study reionization, a period in the history of the universe after the first stars and galaxies appeared. “It’s all very challenging yet very attractive work,” Zhang says.
• January 5, 2022: Magnetic fields are the essential, but often "secret" ingredients of the interstellar medium and the process of making stars. The secrecy shrouding interstellar magnetic fields can be attributed to the lack of experimental probes. 13)
- While Michael Faraday was already probing the link between magnetism and electricity with coils in the early 19th century in the basement of the Royal Institution, astronomers nowadays still cannot deploy coils light-years away.
- Using FAST (Five-hundred-meter Aperture Spherical radio Telescope), an international team led by Dr. LI Di from NAOC (National Astronomical Observatories of the Chinese Academy of Sciences), has obtained accurate magnetic field strength in molecular cloud L1544 - a region of the interstellar medium that seems ready to form stars.
- The team employed the so-called HINSA (HI Narrow Self-Absorption) technique, first conceived by LI Di and Paul Goldsmith based on Arecibo data in 2003. FAST's sensitivity facilitated a clear detection of the HINSA's Zeeman effect. The results suggest that such clouds achieve a supercritical state, i.e., are primed for collapse, earlier than standard models suggest.
Figure 12: The Taurus molecular cloud (grey scale), of which L1544 is a part, is superimposed onto the 2MASS sky image and the field orientation based on Planck data (thin white lines). The HINSA Zeeman spectrum (thick white line) is shown with the fitted Zeeman signature (blue), image credit: NAOC)
- “FAST’s design of focusing radio waves on a cable-driven cabin results in clean optics, which has been vital to the success of the HINSA Zeeman experiment,” said Dr. LI.
- The study was published in Nature on Jan. 5. 14)
- The Zeeman effect — the splitting of a spectral line into several components of frequency in the presence of a magnetic field — is the only direct probe of interstellar magnetic field strength. The interstellar Zeeman effect is small. The frequency shift originating in the relevant clouds is only a few billionths of the intrinsic frequencies of the emitting lines.
- In 2003, the spectra of molecular clouds were found to contain an atomic-hydrogen feature called HINSA, which is produced by hydrogen atoms cooled through collisions with hydrogen molecules. Since this detection was made by the Arecibo telescope, the Zeeman effect for HINSA has been deemed a promising probe of the magnetic field in molecular clouds.
- HINSA has a line strength 5–10 times higher than that of molecular tracers. HINSA also has a relatively strong response to magnetic fields and, unlike most molecular tracers, is robust against astrochemical variations.
- FAST’s HINSA measurements put the magnetic field strength in L1544 at about 4 µGauss, i.e., 6 million times weaker than that of Earth. A combined analysis with quasar (active supermassive blackhole) absorption and hydroxyl emission also revealed a coherent magnetic field structure throughout the cold neutral medium, the molecular envelope, and the dense core, with similar orientation and magnitude.
- Therefore, the transition from magnetic subcriticality to supercriticality — i.e., when the field can and cannot support the cloud against gravity, respectively — occurs in the envelope instead of the core, in contrast with the conventional picture.
- How the interstellar magnetic field dissipates to enable cloud collapse remains an unsolved problem in star formation. The main proposed solution has long been ambipolar diffusion — the decoupling of neutral particles from plasma — in cloud cores.
- The coherence of the magnetic field revealed by the HINSA Zeeman effect means that dissipation of the field occurs during the formation of the molecular envelope, possibly through a different mechanism than ambipolar diffusion.
• October 15, 2021: An international research team led by Prof. LI Di and Dr. WANG Pei from NAOC (National Astronomical Observatories of Chinese Academy of Sciences), caught an extreme episode of cosmic explosions from Fast Radio Burst (FRB) 121102, using FAST (Five-hundred-meter Aperture Spherical radio Telescope). A total of 1,652 independent bursts were detected within 47 days starting August 29, 2019 (UTC). 15)
- It is the largest set of FRB events so far, more than the number reported in all other publications combined. Such a burst set allows for the determination, for the first time, of the characteristic energy and energy distribution of any FRB, thus shedding light on the central engine powering FRBs.
- These results were published in Nature on October 13, 2021. 16)
- FRBs were first detected in 2007. These cosmic explosions can be as short as one-thousandth of a second while producing one year’s worth of the Sun’s total energy output. The origin of FRBs is still unknown. Although even aliens have been considered in models for FRBs, natural causes are clearly favored by the observations. The recent focuses include exotic hyper-magnetized neutron stars, black holes, and cosmic strings left over from the Big Bang.
Figure 13: FAST catches a real pulse from FRB 121102 (image credit: NAOC)
- Scientists have found that a small fraction of FRBs repeat. This phenomenon facilitates follow-up studies, including localization and identification of FRBs’ host galaxies.
- FRB 121102 is the first known repeater and the first well-localized FRB. Scientists have identified its origin in a dwarf galaxy. In addition, this FRB is clearly associated with a persistent radio source. Both clues are crucial to solving the cosmic mystery of FRBs. The behavior of FRB 121102 is hard to predict and commonly described as “seasonal.”
- While testing the FAST FRB backend during the commissioning phase, the team noticed that FRB 121102 was acting up with frequent bright pulses. Between August 29 and October 29, 2019, 1,652 independent burst events were detected in a total of 59.5 hours. While the burst cadence varied during the series, 122 bursts were seen during the peak hour, corresponding to the highest event rate ever observed for any FRB.
- Such high cadence facilitates a statistical study of these FRB bursts. The researchers found a clear characteristic energy of E0= 4.8 x 1037 erg, below which the generation of the bursts became less efficient. The burst energy distribution can be adequately described as bimodal, namely, a log-normal function for low E bursts and a Lorentz function for high E bursts, implying that weaker FRB pulses may be stochastic in nature and the stronger ones involve a ratio between two independent quantities.
Figure 14: The burst rate distribution of isotropic equivalent energy at 1.25 GHz for FRB 121102 (image credit: NAOC)
Figure 15: A “river” of bursts from a galaxy as recorded by the FAST telescope. The burst count and energies are shown in histograms, mimicking the painting “A Vast Land” by WANG Ximeng of the Song Dynasty (image credit: NAOC)
- “The total energy of this burst set already adds up to 3.8% of what is available from a magnetar and no periodicity was found between 1 ms and 1000 s, both of which severely constrains the possibility that FRB 121102 comes from an isolated compact object,” said Dr. WANG.
- More than six new FRBs have been discovered through the Commensal Radio Astronomy FAST Survey (CRAFTS, https://crafts.bao.ac.cn/), including one new 121102-like repeater. “As the world’s largest antenna, FAST’s sensitivity proves to be conducive to revealing intricacies of cosmic transients, including FRBs,” said Prof. LI.
- This project has been part of a long-running collaboration since the commissioning phase of the FAST telescope. Major partner institutions include Guizhou Normal University, University of Nevada Las Vegas, Cornell University, Max-Planck-Institut fuer Radioastronomie, West Virginia University, CSIRO, University of California Berkeley, and Nanjing University.
• April 11, 2021: The opening of China's FAST (Five-hundred-meter Aperture Spherical Radio Telescope) to international scientists could enhance collaboration of scientists from different countries, said Australian astrophysicist Naomi McClure-Griffiths. 17)
- "The telescope is brand new. Many people want to use it, and there's a lot of competition to use the telescope," said McClure-Griffiths in an interview with Xinhua. "As we move into the future, I hope to be able to use it more."
- FAST is in southwest China's Guizhou province. As world's largest filled-aperture and most sensitive radio telescope, it officially opened to the world starting March 31.
- Professor McClure-Griffiths, an astrophysicist and radio astronomer, works at the Research School of Astronomy and Astrophysics in the Australian National University.
- She told Xinhua that she had been to the FAST twice. "I was working in collaboration with a scientist who was the project scientist for FAST, professor Li Di ... and he took me to the telescope to show me how it works. And we've been discussing scientific projects that we could do there."
- She hailed the telescope as an "amazing feat of engineering." "It's absolutely phenomenal," said the professor. "It's big beyond belief and an incredible structure."
- McClure-Griffiths discovered a new spiral arm in the Milky Way galaxy in 2004, and was awarded the Pawsey Medal from the Australian Academy of Science in 2015.
- "I'm interested in how galaxies form and how they evolve, and I think the FAST, with its sensitivity as well as its ability to see fine detail, can image very small areas and will allow us to look at our own galaxy, for example, and study how it is interacting with the medium around it."
- She compared the FAST to a bucket, and said it was good for studying hydrogen in the universe, which is the dominant element that makes up galaxies and shows how they work. "The reason FAST is so good for studying hydrogen is because it's a very big bucket and allows you to see the really weak bits of hydrogen that are in between galaxies."
- FAST could also be used to discover pulsars. To date, FAST has found more than 300 pulsars, and the number is expected to reach 1,000 in the next five years.
- "Pulsars are very compact stars that rotate very quickly," said Professor McClure-Griffiths. "They produce just a very weak signal of radio emission. So the bigger your telescope is, the more likely you are to find them."
- Applications submitted by domestic and overseas scientists to use the FAST will be evaluated by top international experts, which the professor said was good for international study and cooperation.
- Already established in her career, she has had collaborations with Chinese scientists, and had opportunities to visit and understand the telescope. "But for the younger scientists who don't necessarily have the collaborations yet with Chinese astronomers, this (FAST's opening to international scientists) is offering them the opportunity to access the telescope and start to build those collaborations and expand their network."
- McClure-Griffiths said she knew some teams in Australia that look forward to chances when they could access the FAST.
- "It's going to be really exciting to see the results from FAST in the upcoming years," she said.
• March 31, 2021: China's Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the world's largest single-dish and most sensitive radio telescope, will officially open to the world starting 31 March 2021. 18)
- Astronomers worldwide can visit http://fast.bao.ac.cn/proposal_submit to submit their applications for observations, said the National Astronomical Observatories under the Chinese Academy of Sciences in a statement.
- All foreign applications will be evaluated, and the results will be announced on July 20. Observations by international users will begin in August.
Figure 16: Photo taken on March 30, 2021 shows China's Five-hundred-meter Aperture Spherical radio Telescope (FAST) under maintenance at night in southwest China's Guizhou Province. China's FAST, the world's largest single-dish and most sensitive radio telescope, officially opened to the world starting on 31 March 2021 (image credit: Xinhua/Ou Dongqu)
- The statement noted that FAST will provide its research facilities to the world with a more open attitude, offering more observation options for the international astronomical community.
- The project will contribute Chinese wisdom to the construction of a community with a shared future for humanity, and strive to promote international sci-tech development and the progress of human civilization, said the statement.
- Since it started operation, FAST has provided stable and reliable services. It has found 300 pulsars and made breakthroughs in fields like fast radio bursts, a type of powerful radio wave in the sky.
• December 15, 2020: Nestled among the mountains in southwest China, the world's largest radio telescope signals Beijing's ambitions as a global center for scientific research. 19)
- FAST (Five-hundred-meter Aperture Spherical Telescope) — the only significant instrument of its kind after the collapse of another telescope in Puerto Rico this month — is about to open its doors for foreign astronomers to use, hoping to attract the world's top scientific talent.
- The world's second-largest radio telescope, at the Arecibo Observatory in Puerto Rico, was destroyed when its suspended 900-ton receiver platform came loose and plunged 140 meters (450 feet) onto the radio dish below.
- Wang Qiming, chief inspector of FAST's operations and development center, told AFP during a rare visit by the foreign press last week that he had visited Arecibo.
- "We drew a lot of inspiration from its structure, which we gradually improved to build our telescope."
Figure 17: This satellite image shows the Arecibo Observatory in Arecibo, Puerto Rico, after the collapse of its 900-ton receiver platform (image credit: Maxar)
- The Chinese installation in Pingtang, Guizhou province, is up to three times more sensitive than the US-owned one, and is surrounded by a five-kilometer (three-mile) "radio silence" zone where mobile phones and computers are not allowed.
- Work on the FAST began in 2011 and it started full operations in January this year, working mainly to capture the radio signals emitted by celestial bodies, in particular pulsars—rapidly rotating dead stars.
- Closer to home, China has said it will accept requests in 2021 from foreign scientists wishing to carry out measurements.
- "Our scientific committee aims to make FAST increasingly open to the international community," said Wang.
- Sun Jinghai, an engineering manager at the site, predicted there would be a lot of take-up.
- John Dickey, professor of physics at the University of Tasmania in Australia, said the results so far had been impressive.
- "China is certainly a global center for scientific research, at the same level as North America or Western Europe," he said.
- "The community of researchers is as advanced, as creative, and as well organized as in any advanced nation in the world."
- Improvements in scientific innovation have been rapid, said Denis Simon, an expert on Chinese science policy, adding that "China was viewed as an innovation laggard" only a few years ago.
- "More and more discretion and intellectual freedom have been given to the scientific and engineering community to explore new ideas and take bigger risks in the research environment," he said.
- "The risk-averse culture that was once predominant has given way to a more entrepreneurial culture."
- This has included education reforms for new generations of scientists and engineers, he said.
- A sign of the change in China's mentality is that since 2018, foreign scientists have been able to lead state-funded projects.
- "In many ways, the competition between China and the US is about a race for talent—and this race promises to build momentum as the competition between the two countries heats up," he added.
• November 5, 2020: In the vast universe, some extremely strong radio waves occasionally blink, with duration of only milliseconds. Such fast radio bursts were discovered by astronomers in 2007. Puzzling questions arise: Who sent them? What information is conveyed by these radio bursts? 20)
- The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) has revealed some mystery of the fast radio bursts, according to a study published in Nature on October 28. 21)
- More than 30 burst sources have been found until last year. These bursts randomly emerge in the sky, which implies that most of them are not from the Milky Way but outside.
- In 2017, astronomers finally caught a repeating FRB (Fast Radio Burst) source from that several bursts emerge in a few hours. The burst source is found to come from a galaxy 3 billion light years away in the deep Universe.
- The large collecting area makes FAST the most sensitive telescope over the world and the precisely controlled adopt reflectors enables it to make excellent focus and track on celestial targets.
- In the trial open semester in 2019, Dr. Rui Luo, first author of the study, a PhD student from Peking University, hoped to use FAST to see if a radio burst FRB 180301 repeats or not. Fortunately, four bursts were detected during the two-hour observation session on July 16, 2019. This is an exciting result, but no bursts were detected in the four-hour observation on September 11, 2019.
- Considering the position uncertainty of originally reported from the first discovery by the Australian telescope, a research team led by Prof. LI Kejia from Peking University, Prof. Jinlin Han from National Astronomical Observatories of Chinese Academy of Sciences (NAOC), and Prof. Bing Zhang from University of Las Vegas discussed and changed the observations strategy by monitoring the target by using the central beam of 19-beam receiver of the FAST and made the full-polarization data recorded.
- On October 6 and 7, FAST detected 11 bursts in six hours. Totally, FAST detected 15 bursts in 12 hours. The intensity profiles of these bursts are quite different from each other, but definitely they come from one source, similar to the repeating burst source astronomers previously found, with a similar distance of three billion light-years away, and a similar burst rate but much less luminous.
- The most intriguing results come from the careful analysis on the polarization signals of the 11 radio bursts recorded by FAST. Among the 11 bursts detected by FAST, the polarization properties of seven bursts could be well-measured, which showed not only interesting swings of polarization, but also the diversity of swings.
- Such a diverse polarization behavior had never been seen in any radio bursts previously. It means that the bursts are produced in the magnetosphere of compact stars with extremely magnetic fields, such as neutron stars, and disfavors the shock models produced by jets of plasma as proposed by many scientists.
- In the past, polarization signals from only a few of some 30 bursts have been recorded, either showing flat polarization angle swing, i.e. constant polarization direction of radio waves, or variable position angle in one-off bursts. The significance results obtained by FAST observations settle down the debates between the two schools of theoretical models.
- As of today, FAST has conducted intense astronomical observations to many celestial objects, and has discovered more than 230 new pulsars in the last few years.
• July 2, 2020: An international research team led by Dr. CHENG Cheng from the Chinese Academy of Sciences South America Center for Astronomy (CASSACA) observed four extragalactic galaxies by using the FAST 19-beam receiver, and detected the neutral hydrogen line emission from three targets with only five minutes of exposure each. This is the first publication for FAST to detect extragalactic neutral hydrogen. 22) The research paper was published in Astronomy & Astrophysics Letter. 23)
Figure 18: The optical color images of the four galaxies for FAST observation. The red contours are the previous CO observation by ALMA. The white spectra in each panel are the results from FAST (image credit: CASSACA)
- Neutral hydrogen gas is the most extended baryons in galaxies, while cold gas traced by CO is more concentrated to a galaxy center (red contour in Figure 18). "With dynamical measurements of neutral hydrogen and CO, we can estimate the mass distribution of galaxies at different radii," said Dr. CHENG, first author of the study.
- Dynamical masses of these four galaxies estimated from the newly observed neutral hydrogen line were 10 times higher than the observed baryon masses, indicating contribution of dark matter.
- On the other hand, dynamical masses estimated using previous CO observations were equivalent to their observed baryon masses. Therefore, the new FAST observation illustrated its ability of studying dark matter in galaxies using the neutral hydrogen 21cm emission line.
- The FAST observation of these galaxies was an important part of an international research project, the Valparaiso ALMA Line Emission Survey (VALES), led by Prof. Edo Ibar from Valparaiso University in Chile.
- The VALES is a project of observing star forming galaxies using first-class international facilities such as Sloan Digital Sky Survey (SDSS), Herschel Space Observatory, Atacama Large Millimeter Array (ALMA), Atacama Pathfinder Experiment telescope (APEX) and Very Large Telescope (VLT).
- FAST, with the unprecedented sensitivity, provides a unique chance to observe the extra-galactic neutral hydrogen, and therefore has been added to the list of modern astronomical facilities used by this international collaboration.
• March 24, 2020: China's FAST (Five-hundred-meter Aperture Spherical Radio Telescope), the world's largest single-dish radio telescope, has identified 114 new pulsars since its trial operation began in September 2016. 24)
- The gigantic telescope carried out nearly 1,000 hours of observations from 1 January to 23 March 2020, according to the FAST Operation and Development Center of the NAOC/CAS (National Astronomical Observatories of China).
- A pulsar is a highly magnetized, rotating neutron star, which emits two beams of electromagnetic radiation.
- Pulsar observation is an important task for FAST, which can be used to confirm the existence of gravitational radiation and black holes, and help solve many other major questions in physics.
- FAST is also in charge of the exploration of interstellar molecules and interstellar communication signals.
- Located in a naturally deep and round karst depression in southwest China's Guizhou Province, FAST is believed to be the world's most sensitive radio telescope. It started formal operation on 11 January 2020 after it passed a national assessment.
• October 11, 2019: For more than a month, Chinese scientists have been studying mysterious fast radio bursts coming from space. China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the world’s largest radio telescope, has now picked up more of these radio pulses that have baffled astronomers for more than a decade. 25)
- Chinese astrophysicists studying FRBs (Fast Radio Bursts) have reported dozens of new signals. A mysterious radio pulse, known as the FRB121102, was first detected by FAST in August, and since then the signals have periodically reappeared. FRB121102 was already known to scientists and was first picked up in 2012 at the Arecibo Observatory in Puerto Rico and then again in 2015.
- But what are these fast radio bursts and why are they so difficult to study?
- The first fast radio bursts were picked up in 2001, Yongfeng Huang, a professor at the School of Astronomy and Space Sciences of Nanjing University, said.
- “Back in 2001, when astronomers were analyzing archival data recorded by radio telescopes, they suddenly came across the strange phenomenon, when for a short time, the telescopes detected very powerful wave bursts. Although their duration was only one-thousandth of a second, they were of extreme intensity,” Prof. Huang explains.
- But it wasn’t until 2007 that FRBs were first discovered. “They were called fast radio bursts. In other words, these are unidentified radio wave pulses coming from the depths of space,” the scholar added.
- FRBs are hard to track, and scientists disagree on the nature and origin of these signals.
- “Previously, people were unaware of these phenomena. Even astronomers could not explain their nature. At first, it was believed that this is interference caused by radiation from our planet, but then Australian scientists in the course of research found that the source of the radio bursts is located somewhere in space,” Prof. Huang said.
- Among the hypotheses on their origin, there was a version postulating the alien nature of the signals. However, there is still no definitive answer, and in 2010, signals from the Earth were mistaken for FRB.
- “Some authoritative scientists believe that the bursts may well come from the solar engines of extraterrestrial civilizations. However, not many astronomers adhere to that theory,” the Chinese professor stated.
- At the moment, it’s hard to pinpoint the source of these rapid pulses of energy. According to Prof. Huang, it will take a lot of time to establish it: “I, like many astronomers, believe these bursts have only been known to science for ten years - this is a very short time in astronomy. Besides, tracking and studying these FRBs is quite difficult, as they occur and disappear extremely quickly, while seldom appearing in the same place. We can only observe their radiation backgrounds without the ability to study other parameters. Therefore, we won’t be able to establish the source of these bursts soon. At least not earlier than 10-20 years from now.”
- In 2019, scientists spotted another pulse from FRB121102 on 30 August. After that, the telescope recorded dozens of repeating signals from the FBR over several days. On 3 September alone, there were more than 20 pulses detected. But the number of FRBs detected so far it still too small to make any conclusions the scholar said.
- “In all that time, astronomers have detected about a hundred such bursts. Most of them did not occur where they were previously detected. Therefore, it is difficult to determine the source of the phenomenon.”
- Studying them is also not easy because these bursts are almost impossible to locate due to the nature of their appearances.
- “According to the roughest estimates, there are about 10,000 of them daily, but our telescopes do not record them in such numbers. The most curious thing is that we can’t say if there is any connection between the detected bursts since we are not able to track them for a sufficiently long period. In any case, repeated fast radio bursts once again confirm that this is not radio interference from Earth, but something reaching to us from outer space,” Prof. Huang concludes.
• September 24, 2019: The world’s largest single-dish radio observatory is preparing to open to astronomers around the world, ushering in an era of exquisitely sensitive observations that could help in the hunt for gravitational waves and probe the mysterious fleeting blasts of radiation known as fast radio bursts. 26)
- FAST (Five-hundred-meter Aperture Spherical Radio Telescope) in southern China has just passed a series of technical and performance assessments, and the Chinese government is expected to give the observatory the final green light to begin full operations at a review meeting scheduled for next month. “We do not see any roadblocks for the remaining transition,” says Di Li, the chief scientist of FAST. “I feel both excited and relieved.”
- The complex project has not been without challenges — it has a radical design and initially struggled to attract staff, in part because of its remote location. But the pay-off for science will be immense. FAST will collect radio waves from an area twice the size of the next-largest single-dish telescope, the Arecibo Observatory in Puerto Rico.
- The Chinese observatory’s massive size means that it can detect extremely faint radio-wave whispers from an array of sources across the Universe, such as the spinning cores of dead stars, known as pulsars, and hydrogen in distant galaxies. It will also explore a frontier in radioastronomy — using radio waves to locate exoplanets, which may harbour extraterrestrial life.
- Since testing began in 2016, only Chinese scientists have been able to lead projects studying the telescope’s preliminary data. But now, observation time will be accessible to researchers from around the world, says Zhiqiang Shen, director of the Shanghai Astronomical Observatory and co-chair of the Chinese Academy of Sciences’ FAST supervisory committee.
- “I’m super excited to be able to use the telescope,” says Maura McLaughlin, a radioastronomer at West Virginia University in Morgantown, who wants to use FAST to study pulsars, including hunting for them in galaxies outside the Milky Way, that are too faint to see with current telescopes.
- During the testing phase, the telescope discovered more than 100 pulsars.
Eye in the sky
- The 1.2-billion-yuan (US$171-million) telescope, also known as Tianyan or ‘Eye of Heaven’, took half a decade to build in the remote Dawodang depression in the Guizhou province of southwest China. Its 500-meter-wide dish is made up of around 4,400 individual aluminium panels that more than 2,000 mechanical winches tilt and manoeuvre to focus on different areas of the sky. Although it sees less of the sky than some other cutting-edge radio telescopes, and has lower resolution than multidish arrays, FAST’s size makes it uniquely sensitive, says Li.
- In August and September, the instrument detected hundreds of bursts from a repeating fast radio burst (FRB) source known as 121102. Many of these bursts were too faint to be perceived by other telescopes, says Li. “This is very exciting news,” says Yunfan Gerry Zhang, who studies FRBs at the University of California, Berkeley. No one knows what causes the mysterious bursts, but “the more pulses we have, the more we can learn about them”, he says.
- FAST examines only a tiny fraction of the sky at any one time, making it unlikely to discover many new FRBs, which are fleeting and occur in seemingly random locations. But the telescope’s “impressive sensitivity” will be useful for following up on sources in detail, says Laura Spitler, an astronomer at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Repeat observations could allow scientists to learn about the environment from which an FRB emerged, and to determine whether the blasts vary in energy or recur with any set pattern.
- FAST will also boost the efforts of an international collaboration that is trying to spot ripples in space-time as they sweep through the Galaxy, says McLaughlin. The International Pulsar Timing Array is using radio telescopes around the world to monitor the regular emissions from pulsars, looking for distortions that would reveal the passing of these low-frequency gravitational waves. By the 2030s, FAST should have racked up enough sensitive measurements to study individual sources of such waves, such as collisions of supermassive black holes, says McLaughlin. “That’s where FAST is really going to shine,” she says.
- Li says that he is particularly excited about the study of planets outside the Solar System. No exoplanets have yet been conclusively detected by their radio emissions, but FAST’s ability to spot faint, polarized waves might allow it to find the first examples, says Li. Polarized radio signals might come from planets with magnetic fields that, if similar to the one on Earth, could protect potential sources of life against radiation and keep the planets’ atmospheres attached.
- Identifying a planet in FAST’s wide beam is a challenge, because they are so faint and small. But Li’s team wants to boost the telescope’s performance by adding 36 dishes, each 5 meters wide. Although the dishes are relatively cheap, off-the-shelf products, together they will improve FAST’s spatial resolution by 100 times, he says.
- Li hopes that FAST’s telescope operations will soon move from near the remote site to a $23-million data-processing centre being built in the city of Guiyang. He expects that the move to a major city will help attract more technical and engineering staff.
- Now the team’s biggest hurdle is working out how to store and process the enormous amount of data that the telescope will churn out. The team are negotiating with the Chinese government to get additional funding for more data storage. “A successful review will definitely help,” he says.
• September 10, 2019: Chinese astronomers have detected repeated FRBs (Fast Radio Bursts) - mysterious signals believed to be from a source about 3 billion light years from Earth - with the largest and most sensitive radio telescope ever built. 27)
- Scientists detected the signals with the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) and they are carefully cross-checking and processing them, according to researchers at the NAOC (National Astronomical Observatories of the Chinese Academy of Sciences).
- FRBs are the brightest bursts known in the universe. They are called "fast" because these blips are very short, only several milliseconds in duration. But there is no reasonable explanation for their origin.
- The detection of the repeated bursts might help shed light on the origin and physical mechanisms of FRBs, said researchers.
- Chinese scientists have installed a highly sensitive FRB backend on a 19-beam receiver on the giant telescope, and used it to observe an FRB source named FRB121102, which was first discovered by the Arecibo Observatory in 2015.
- From late August to the beginning of September, more than 100 bursts were detected from FRB121102, the highest number of bursts ever detected so far.
- The FRB backend system has high-efficiency real-time pulse capture capability, and can observe in parallel with most observation tasks. It will play an important role in the discovery of new FRBs, improving the position accuracy and capturing the high-resolution absorption lines generated by FRBs in real time, researchers said.
- Given the significance of this source and its now apparent active state, FAST is carrying out more monitoring. Chinese astronomers encouraged counterparts in other countries to conduct more observations with their facilities.
- Located in a naturally deep and round karst depression in southwest China's Guizhou Province, FAST was completed in September 2016 and is due to start regular operations this month.
- Astronomers from more than 10 countries and regions are making observation plans for FAST in order to best apply the unprecedented power of the telescope, going beyond what has been done by other telescopes in the past.
- They have proposed ambitious observation objectives for the telescope, such as gravitational waves, exoplanets, ultra-high-energy cosmic rays and interstellar matter, to advance human knowledge of astronomy, astrophysics and fundamental physics.
• January 22, 2019: No cell phones, no digital cameras, and no smart wearable devices - the Guizhou provincial government has revised a regulation to keep the noise down and prevent human activities from affecting the world's largest telescope. The blanket ban is enforced on radio equipment and electromagnetic gadgets, including tablets, speakers and drones, in the core silence zone of the Five-hundred-meter Aperture Spherical Telescope (FAST). 28)
- The total quiet area of FAST includes a 5-km radius as the core zone, 5 km to 10 km radius as the intermediate zone and a peripheral zone that covers 10 km to 30 km in radius.
- Visitors and tourists who enter the quiet area shall abide by the regulation to minimize confusing operations of the telescope.
- Since FAST began its trial operation in 2016, it has seen a tourist surge. A regulation in 2013 did not include details on how to prevent human activities from interfering with the operation of the giant dish.
- The new regulation is more specific on the management of visitors, delineates responsibilities for regulators, orders warning signposts to be established and requires local authorities to engage the public in maintaining an interference-free environment. It raised the maximum fine for those who break the rules from 5,000 yuan (about 734 U.S. dollars) to 200,000 yuan.
- The regulation is expected to be effective starting from April 1, 2019.
- Radio telescopes work by tracking and reading faint energy waves from stars and gases, so they have to be located in sparsely populated areas to avoid electromagnetic interference. The giant dish, which is 500 meters in diameter, has found 44 pulsars.
- The telescope will start formal operation and open to Chinese astronomers this year.
• September 27, 2018: His eyes brimming with excitement, seven-year-old Wang Jun ran to an exhibition stand to pick up a pair of headphones and started listening, leaving his father behind. "The Sound of Pulsar Stars collected by FAST," read a sign next to the stand. FAST (Five-hundred-meter Aperture Spherical Radio Telescope) is the world's largest single-dish radio telescope and was set up two years ago on this day in southwest China's Guizhou Province. It helps scientists understand the universe by receiving and recording pulsar and interstellar signals from extraterrestrial sources. 29)
- Engineers and astronomers continuously try to perfect the telescope, making improvements to allow it to see farther into space. Meanwhile, those who visit the telescope find themselves in awe of the giant dish and its ability to lead to breakthrough discoveries.
- Sensitive Giant: Since its trial operation in 2016, FAST has found some 50 stars which bear features similar to pulsars, with 44 confirmed, according to scientists of NAOC (National Astronomical Observatories of China).
- Pulsar observation can be used to conduct research on gravitational waves, black holes and to help solve many other major questions in physics.
- "We are still improving the system," said Jiang Peng, chief engineer of FAST with NAOC, during Xinhua's recent tour to the FAST observatory. "Now we have met many goals previously set for the telescope."
- The sensitivity of a telescope is the minimum brightness that it can detect. The lower the number, the farther a telescope can see. In FAST's case, Jiang's team cut the number by 20 percent in the last two years, making it arguably the world's most sensitive telescope.
- They have also extended its annual observation time from around 700 hours to more than 1,000 hours, which means more data for scientists to analyze.
- The telescope will start formal operation and open to Chinese astronomers in 2019, according to NAOC. "We often say the telescope was almost usable two years ago; now it is usable, and our goal is to make it good to use," Jiang said.
- Silent Tourism: The state-of-art technology and the spectacle of the giant dish have become a magnet for tourists over the years.
- "My son is interested in science and aliens," said Wang Lifa, Wang Jun's father. "We are here to satisfy his imagination." Wang drove six hours from a neighboring province to Kedu township of Pingtang County, around 15 km away from the mega-telescope. Visitors gather here before they go for a closer view of the giant dish.
- Tourism took off in the once-impoverished town surrounded by lush forests as wider roads, fancy hotels and bustling shops have sprung up. In the first half of 2018, Pingtang County received 5.13 million visitors, up 40.58 percent. The tourists brought in 550 million yuan (around 80 million U.S. dollars) for the small county, according to the local newspaper Qiannan Daily.
- The tourist surge has also stoked concerns that it might affect the telescope whose probe results can be compromised by radio signals from electronic devices carried by tourists.
- Around FAST, a 30-km perimeter was set up as a "silent zone" where the frequencies and radio power are strictly limited.
- To view the telescope, tourists go to a core zone with a radius of five km around the FAST. Restrictions are even more extreme in that area: no phones, laptops or cameras. Even the GPS system on the ferry to the site is disabled.
- The local government has also developed plans to curb tourists. Last Thursday, the scenic spot stopped selling onsite tickets for the ferry buses to FAST and museums and moved the operation to an online booking website. The local government restricted the number of tourists to the site to 2,000 per day. "So far, the protection against signal interferences in the core zone has turned out to be effective," said Jiang, the FAST chief engineer.
• April 30, 2018: China's FAST, still under commissioning, discovered a radio millisecond pulsar (MSP) coincident with the unassociated gamma-ray source 3FGL J0318.1+0252 in the Fermi Large Area Telescope (LAT) point-source list. This is another milestone of FAST. Fast has discovered more than 20 new pulsars so far. This first MSP discovery was made by FAST on Feb. 27 and later confirmed by the Fermi-LAT team in reprocessing of Fermi data on April 18th. 30) 31)
- The newly discovered pulsar, now named PSR J0318+0253, is confirmed to be isolated through timing of gamma-ray pulsations. This discovery is the first result from the FAST-Fermi LAT collaboration outlined in a MoU signed between the FAST team and Fermi-LAT team.
- "This discovery demonstrated the great potential of FAST in pulsar searching, highlighting the vitality of the large aperture radio telescope in the new era," said Kejia Lee, scientist at the Kavli Institute of Astronomy and Astrophysics, Peking University.
- Radio follow-up of Fermi-LAT unassociated sources is an effective way for finding new pulsars. Previous radio observations, including three epochs with Arecibo in June 2013, failed to detect the MSP. In an one-hour tracking observation with the FAST ultra-wide band receiver, the radio pulses toward 3FGL J0318.1+0252 were detected with a spin period of 5.19 milliseconds, an estimated distance of about 4 thousand light-years, and as potentially one of the faintest radio MSPs.
- A millisecond pulsar is a special kind of neutron stars that rotate hundreds of times per second. It is not only expected to play an important role in understanding the evolution of neutron stars and the equation of state of condense matter, but also can be used to detect low-frequency gravitational waves.
- The pulsar timing array (PTA) attempts to detect low-frequency gravitational waves from merging supermassive black holes using the long-term timing of a set of stable millisecond pulsars. Pulsar search is the basis of gravitational wave detection through PTAs.
- "The international radio-astronomy community is excited about the amazing FAST telescope, already showing its power in these discoveries. FAST will soon discover a large number of millisecond pulsars and I am looking forward to seeing FAST's contribution to gravitational wave detection," said George Hobbs, scientist of the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia and member of the Gravitational Wave International Committee (GWIC).
- FAST will be under commissioning until it reaches the designed specifications and becomes a Chinese national facility.
Figure 19: Radio follow-up of Fermi-LAT unassociated sources is an effective way for finding new pulsars. Previous radio observations, including three epochs with Arecibo in June 2013, failed to detect the MSP. In an one-hour tracking observation with the FAST ultra-wide band receiver, the radio pulses toward 3FGL J0318.1+0252 were detected with a spin period of 5.19 milliseconds, an estimated distance of about 4 thousand light-years, and as potentially one of the faintest radio MSPs (image credit: NAOC/CAS)
• March 15, 2018: China's FAST (Five-hundred-meter Aperture Spherical Radio Telescope), the world’s largest single-dish radio telescope, has discovered 11 new pulsars since its September 2016 inauguration, according to China’s National Astronomical Observatories. 32)
• On September 24, 2016, China officially put into operation the world's largest single-aperture telescope. Scientists said they welcome their foreign counterparts for space research after first debugging the facility to ensure its best optimal performance. 33)
- Chinese President Xi Jinping sent a congratulatory letter to scientists, engineers and builders as the world's largest radio telescope was officially put into use in southwest China's Guizhou province on Sunday. 34)
- A launch ceremony was held in Pingtang County, Guizhou, for the Five-hundred-meter Aperture Spherical Telescope (FAST).
- Its launch is significant for China to achieve major breakthroughs in frontier scientific fields and to expedite innovation-driven growth, Xi said, adding astronomy is crucial to propelling scientific progress and innovation.
• On July 3, 2016, with the command given by Yan Jun, the FAST project general manager, National Astronomical Observatory director, the last reflection surface element was slowly lifting and moving to air and finally fell to the designated location on cable net. 35)
- The active reflectors are an important part of the FAST telescope, a total of 4450 blocks of the reflective panel unit, including 4273 basic types and 177 special types. Reflector unit length is 10.4 to 12.4 meters, each unit has a mass of 427 to 482.5 kg, the thickness is about 1.3 mm.
- On August 2, 2015, the FAST reflector unit lifting project started construction. A block of reflector units on the ground after assembling, measurement, inspection and strict steps form qualified units, through the tower crane, transport vehicles, cable crane and so on a series of complex high process will be every piece of units shipped to the designated location to install. To overcome the large scale and high precision assembling construction difficulties and large span, position higher hoisting construction problems, after 11 months of effort, an area of nearly 30 football fields of the reflecting surface by a block reflector units gradually laying was completed.
- Reflective surface engineering is also the last FAST equipment engineering, and its successful completion marks the successful completion of the main project of FAST.
- The reflector unit design, manufacturing and assembling tasks were done by the China Electronics Technology Group Corporation No. 54 of the consortium with Zhejiang southeast space frame Co., Ltd, while the reflector unit lifting tasks were undertaken by the Wuchang Heavy Engineering Co., Ltd..
Figure 20: On July 3, 2016, all 4450 reflector panels have been installed on FAST (image credit: NAOC/CAS)
Figure 21: Installing the last reflector panel (image credit: NAOC/CAS)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
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