Radio astronomy

Chart on which Jocelyn Bell Burnell first recognised evidence of a pulsar, in 1967 (exhibited at Cambridge University Library)

Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources. In the 1860s, James Clerk Maxwell's equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. Several attempts were made to detect radio emission from the Sun including an experiment by German astrophysicists Johannes Wilsing and Julius Scheiner in 1896 and a centimeter wave radiation apparatus set up by Oliver Lodge between 1897 and 1900. These attempts were unable to detect any emission due to technical limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable.[1]

Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions. Using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun exactly, but instead repeating on a cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend, astrophysicist and teacher Albert Melvin Skellett, who pointed out that the time between the signal peaks was the exact length of a sidereal day; the time it took for "fixed" astronomical objects, such as a star, to pass in front of the antenna every time the Earth rotated.[2] By comparing his observations with optical astronomical maps, Jansky eventually concluded that the radiation source peaked when his antenna was aimed at the densest part of the Milky Way in the constellation of Sagittarius.[3] He concluded that since the Sun (and therefore other stars) were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy.[2] (Jansky's peak radio source, one of the brightest in the sky, was designated Sagittarius A in the 1950s and, instead of being galactic "gas and dust", was later hypothesized to be emitted by electrons in a strong magnetic field. Current thinking is that these are ions in orbit around a massive Black hole at the center of the galaxy at a point now designated as Sagitarius A*. The asterisk indicates that the particles at Sagitarius A are ionized.)[4][5][6][7] Jansky announced his discovery in 1933. He wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of flux density, the jansky (Jy), after him.

Grote Reber was inspired by Jansky's work, and built a parabolic radio telescope 9m in diameter in his backyard in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies.[8] On February 27, 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun.[9] Later that year George Clark Southworth,[10] at Bell Labs like Jansky, also detected radiowaves from the sun. Both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first.[11] Several other people independently discovered solar radiowaves, including E. Schott in Denmark[12] and Elizabeth Alexander working on Norfolk Island.[13][14][15][16]

The Robert C. Byrd Green Bank Telescope (GBT) in West Virginia, United States is the world's largest fully steerable radio telescope.

At Cambridge University, where ionospheric research had taken place during World War II, J.A. Ratcliffe along with other members of the Telecommunications Research Establishment that had carried out wartime research into radar, created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied.

This early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis. The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used the Cambridge Interferometer to map the radio sky, producing the Second (2C) and Third (3C) Cambridge Catalogues of Radio Sources.[17]

First 7-metre ESO/NAOJ/NRAO ALMA Antenna.[18]

Radio astronomers use different techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze its emission. To "image" a region of the sky in more detail, multiple overlapping scans can be recorded and pieced together in a mosaic image. The type of instrument used depends on the strength of the signal and the amount of detail needed.

Observations from the Earth's surface are limited to wavelengths that can pass through the atmosphere. At low frequencies, or long wavelengths, transmission is limited by the ionosphere, which reflects waves with frequencies less than its characteristic plasma frequency. Water vapor interferes with radio astronomy at higher frequencies, which has led to building radio observatories that conduct observations at millimeter wavelengths at very high and dry sites, in order to minimize the water vapor content in the line of sight. Finally, transmitting devices on earth may cause radio-frequency interference. Because of this, many radio observatories are built at remote places.

Radio telescopes

Main article: Radio telescope

An optical image of the galaxy M87 (HST), a radio image of same galaxy using Interferometry (Very Large Array – VLA), and an image of the center section (VLBA) using a Very Long Baseline Array (Global VLBI) consisting of antennas in the US, Germany, Italy, Finland, Sweden and Spain. The jet of particles is suspected to be powered by a black hole in the center of the galaxy.

Radio telescopes may need to be extremely large in order to receive signals with low signal-to-noise ratio. Also since angular resolution is a function of the diameter of the "objective" in proportion to the wavelength of the electromagnetic radiation being observed, radio telescopes have to be much larger in comparison to their optical counterparts. For example, a 1-meter diameter optical telescope is two million times bigger than the wavelength of light observed giving it a resolution of roughly 0.3 arc seconds, whereas a radio telescope "dish" many times that size may, depending on the wavelength observed, only be able to resolve an object the size of the full moon (30 minutes of arc).

Very-long-baseline interferometry

Main article: Very-long-baseline interferometry

The Mount Pleasant Radio Telescope is the southernmost antenna used in Australia's VLBI network

Beginning in the 1970s, improvements in the stability of radio telescope receivers permitted telescopes from all over the world (and even in Earth orbit) to be combined to perform very-long-baseline interferometry. Instead of physically connecting the antennas, data received at each antenna is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. At that later time, the data is correlated with data from other antennas similarly recorded, to produce the resulting image. Using this method it is possible to synthesise an antenna that is effectively the size of the Earth. The large distances between the telescopes enable very high angular resolutions to be achieved, much greater in fact than in any other field of astronomy. At the highest frequencies, synthesised beams less than 1 milliarcsecond are possible.

The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes located across North America) and the European VLBI Network (telescopes in Europe, China, South Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed together producing increased sensitivity. This is referred to as Global VLBI. There are also a VLBI networks, operating in Australia and New Zealand called the LBA (Long Baseline Array),[19] and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).[20]

Since its inception, recording data onto hard media was the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was originally pioneered in Japan, and more recently adopted in Australia and in Europe by the EVN (European VLBI Network) who perform an increasing number of scientific e-VLBI projects per year.[21]

A radio image of the central region of the Milky Way galaxy. The arrow indicates a supernova remnant which is the location of a newly discovered transient, bursting low-frequency radio source GCRT J1745-3009.

Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including pulsars, quasars[22] and radio galaxies. This is because radio astronomy allows us to see things that are not detectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.

The cosmic microwave background radiation was also first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the Sun and solar activity, and radar mapping of the planets.

Other sources include:

• Sun
• Jupiter
• Sagittarius A, the galactic center of the Milky Way, with one portion Sagittarius A* thought to be a radio wave emitting supermassive black hole
• Active galactic nuclei and pulsars have jets of charged particles which emit synchrotron radiation
• Merging galaxy clusters often show diffuse radio emission[23]
• Supernova remnants can also show diffuse radio emission; pulsars are a type of supernova remnant that shows highly synchronous emission.
• The cosmic microwave background is blackbody radio/microwave emission

Antenna 100 m of the Radio telescope Effelsberg, Germany

Antenna 70 m of the Goldstone Deep Space Communications Complex, California

Antenna 110m of the Green Bank radio telescope, USA

Radio astronomy service (also: radio astronomy radiocommunication service) is, according to Article 1.58 of the International Telecommunication Union's (ITU) Radio Regulations (RR),[24] defined as "A radiocommunication service involving the use of radio astronomy". Subject of this radiocommunication service is to receive radio waves transmitted by astronomical or celestial objects.

• Atacama Large Millimeter Array
• Channel 37
• Gamma-ray astronomy
• Infrared astronomy
• Radar astronomy
• X-ray astronomy
• Waves (Juno) (radio instrument on the Juno Jupiter orbiter)
• Radio Galaxy Zoo

Wikimedia Commons has media related to Radio astronomy.

• nrao.edu National Radio Astronomy Observatory
• The History of Radio Astronomy * Reber Radio Telescope – National Park Services
• Radio Telescope Developed – a brief history from NASA Goddard Space Flight Center
• Society of Amateur Radio Astronomers
• Visualization of Radio Telescope Data Using Google Earth
• UnwantedEmissions.com A general reference for radio spectrum allocations, including radio astronomy.
• Improving Radio Astronomy Images by Array Processing
• What is Radio Astronomy – Radioastrolab