astronomical radio source

An astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe.

History

In 1932, American physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source in the center of our galaxy. Jansky was studying the origins of radio frequency interference for Bell Laboratories. He found "...a steady hiss type static of unknown origin", which eventually he concluded had an extraterrestrial origin. This was the first time that radio waves were detected from outer space. The first radio sky survey was conducted by Grote Reber and was completed in 1941. In the 1970s, some stars in our galaxy were found to be radio emitters, one of the strongest being the unique binary MWC 349.

Sources: solar system

The Sun

As the nearest star, the Sun is the brightest radiation source in most frequencies, down to the radio spectrum at 300 MHz (1 m wavelength). When the Sun is quiet, the galactic background noise dominates at longer wavelengths. During geomagnetic storms, the Sun will dominate even at these low frequencies.

Jupiter

Oscillation of electrons trapped in the magnetosphere of Jupiter produce strong radio signals, particularly bright in the decimeter band.

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on Jupiter's moon Io injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output.

Ganymede

In 2021 news outlets reported that scientists, with the Juno spacecraft that orbits Jupiter since 2016, detected an FM radio signal from the moon Ganymede at a location where the planet's magnetic field lines connect with those of its moon. According to the reports these were caused by cyclotron maser instability and were similar to both WiFi-signals and Jupiter's radio emissions. A study about the radio emissions was published in September 2020 but did not describe them to be of FM nature or similar to WiFi signals.

Sources: Galactic

The Galactic Center

The center of the Milky Way was the first radio source to be detected. It contains a number of radio sources, including Sagittarius A, the compact region around the supermassive black hole, Sagittarius A*, as well as the black hole itself. When flaring, the accretion disk around the supermassive black hole lights up, detectable in radio waves.

In the 2000s, three Galactic Center Radio Transients (GCRTs) were detected: GCRT J1746–2757, GCRT J1745–3009, and
GCRT J1742–3001. In addition, ASKAP J173608.2-321635, which was detected six times in 2020, may be a fourth GCRT.

;Region around the Galactic Center
In 2021, astronomers reported the detection of peculiar, highly circularly polarized intermittent radio waves from near the galactic center whose unidentified source could represent a new class of astronomical objects with a GCRT so far not "fully explain ngthe observations".

Supernova remnants

Supernova remnants often show diffuse radio emission. Examples include Cassiopeia A, the brightest extrasolar radio source in the sky, and the Crab Nebula.

Neutron stars

Pulsars

Supernovae sometimes leave behind dense spinning neutron stars called pulsars. They emit jets of charged particles which emit synchrotron radiation in the radio spectrum. Examples include the Crab Pulsars (dense central cores of extremely distant galaxies) were both discovered by radio astronomers. In 2003 astronomers using the Parkes radio telescope discovered two pulsars orbiting each other, the first such system known.

Rotating Radio Transient (RRAT) Sources

Rotating radio transients (RRATs) are a type of neutron stars discovered in 2006 by a team led by Maura McLaughlin from the Jodrell Bank Observatory at the University of Manchester in the UK. RRATs are believed to produce radio emissions which are very difficult to locate, because of their transient nature. Early efforts have been able to detect radio emissions (sometimes called RRAT flashes) for less than one second a day, and, like with other single-burst signals, one must take great care to distinguish them from terrestrial radio interference. Distributing computing and the Astropulse algorithm may thus lend itself to further detection of RRATs.

Star forming regions

Short radio waves are emitted from complex molecules in dense clouds of gas where stars are giving birth.

Spiral galaxies

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