Astronomical radio source

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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.

Contents

History

In 1932, American physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source in the center of the Milky Way 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. [1] The first radio sky survey was conducted by Grote Reber and was completed in 1941. In the 1970s, some stars in the Milky Way were found to be radio emitters, one of the strongest being the unique binary MWC 349. [2]

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. [3]

Jupiter

Magnetosphere of Jupiter Jupiter's Magnetosphere animation.png
Magnetosphere of 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. [4]

Ganymede

Jupiter's moon Ganymede Ganymede JunoGill 2217.jpg
Jupiter's moon 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. [5] [6] A study about the radio emissions was published in September 2020 [7] but did not describe them to be of FM nature or similar to WiFi signals.[ clarification needed ]

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. [8] In addition, ASKAP J173608.2-321635, which was detected six times in 2020, may be a fourth GCRT. [9] [8]

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[ing] the observations". [10] [11] [8]

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

Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates. Pulsar schematic.svg
Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates.

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 Pulsar, the first pulsar to be discovered. Pulsars and quasars (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. [12] Early efforts have been able to detect radio emissions (sometimes called RRAT flashes) [13] 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 contain clouds of neutral hydrogen and carbon monoxide which emit radio waves. The radio frequencies of these two molecules were used to map a large portion of the Milky Way galaxy. [14]

Sources: extra-galactic

Radio galaxies

Many galaxies are strong radio emitters, called radio galaxies. Some of the more notable are Centaurus A and Messier 87.

Quasars (short for "quasi-stellar radio source") were one of the first point-like radio sources to be discovered. Quasars' extreme redshift led us to conclude that they are distant active galactic nuclei, believed to be powered by black holes. Active galactic nuclei have jets of charged particles which emit synchrotron radiation. One example is 3C 273, the optically brightest quasar in the sky.

Merging galaxy clusters often show diffuse radio emission. [15]

Cosmic microwave background

The cosmic microwave background is blackbody background radiation left over from the Big Bang (the rapid expansion, roughly 13.8 billion years ago, [16] that was the beginning of the universe.

Extragalactic pulses - Fast Radio Burst

D. R. Lorimer and others analyzed archival survey data and found a 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud. They reported that the burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. In a recent paper, they argue that current models for the free electron content in the universe imply that the burst is less than 1 gigaparsec distant. The fact that no further bursts were seen in 90 hours of additional observations implies that it was a singular event such as a supernova or coalescence (fusion) of relativistic objects. [17] It is suggested that hundreds of similar events could occur every day and, if detected, could serve as cosmological probes. Radio pulsar surveys such as Astropulse-SETI@home offer one of the few opportunities to monitor the radio sky for impulsive burst-like events with millisecond durations. [18] Because of the isolated nature of the observed phenomenon, the nature of the source remains speculative. Possibilities include a black hole-neutron star collision, a neutron star-neutron star collision, a black hole-black hole collision, or some phenomenon not yet considered.

In 2010 there was a new report of 16 similar pulses from the Parkes Telescope which were clearly of terrestrial origin, [19] but in 2013 four pulse sources were identified that supported the likelihood of a genuine extragalactic pulsing population. [20]

These pulses are known as fast radio bursts (FRBs). The first observed burst has become known as the Lorimer burst. Blitzars are one proposed explanation for them.

Sources: not yet observed

Primordial black holes

According to the Big Bang Model, during the first few moments after the Big Bang, pressure and temperature were extremely great. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe, a primordial black hole would be stable, persisting to the present.

One goal of Astropulse is to detect postulated mini black holes that might be evaporating due to "Hawking radiation". Such mini black holes are postulated [21] to have been created during the Big Bang, unlike currently known black holes. Martin Rees has theorized that a black hole, exploding via Hawking radiation, might produce a signal that's detectable in the radio. The Astropulse project hopes that this evaporation would produce radio waves that Astropulse can detect. The evaporation wouldn't create radio waves directly. Instead, it would create an expanding fireball of high-energy gamma rays and particles. This fireball would interact with the surrounding magnetic field, pushing it out and generating radio waves. [22]

ET

Previous searches by various "search for extraterrestrial intelligence" (SETI) projects, starting with Project Ozma, have looked for extraterrestrial communications in the form of narrow-band signals, analogous to our own radio stations. The Astropulse project argues that since we know nothing about how ET might communicate, this might be a bit closed-minded. Thus, the Astropulse Survey can be viewed[ by whom? ] as complementary to the narrow-band SETI@home survey as a by-product of the search for physical phenomena.[ citation needed ]

Other undiscovered phenomena

Explaining their discovery in 2005 of a powerful bursting radio source, NRL astronomer Dr. Joseph Lazio stated: [23] "Amazingly, even though the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, very little has been done to look for radio bursts, which are often easier for astronomical objects to produce." The use of coherent dedispersion algorithms and the computing power provided by the SETI network may lead to discovery of previously undiscovered phenomena.

See also

Related Research Articles

<span class="mw-page-title-main">Neutron star</span> Collapsed core of a massive star

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially metal-rich. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

<span class="mw-page-title-main">Quasar</span> Active galactic nucleus containing a supermassive black hole

A quasar is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way. Quasars are usually categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

Cosmic noise, also known as galactic radio noise, is not actually sound, but a physical phenomenon derived from outside of the Earth's atmosphere. It can be detected through a radio receiver, which is an electronic device that receives radio waves and converts the information given by them to an audible form. Its characteristics are comparable to those of thermal noise. Cosmic noise occurs at frequencies above about 15 MHz when highly directional antennas are pointed toward the Sun or other regions of the sky, such as the center of the Milky Way Galaxy. Celestial objects like quasars, which are super dense objects far from Earth, emit electromagnetic waves in their full spectrum, including radio waves. The fall of a meteorite can also be heard through a radio receiver; the falling object burns from friction with the Earth's atmosphere, ionizing surrounding gases and producing radio waves. Cosmic microwave background radiation (CMBR) from outer space is also a form of cosmic noise. CMBR is thought to be a relic of the Big Bang, and pervades the space almost homogeneously over the entire celestial sphere. The bandwidth of the CMBR is wide, though the peak is in the microwave range.

<span class="mw-page-title-main">Radio astronomy</span> Subfield of astronomy that studies celestial objects at radio frequencies

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.

<span class="mw-page-title-main">Magnetar</span> Type of neutron star with a strong magnetic field

A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

<span class="mw-page-title-main">Kardashev scale</span> Measure of a civilizations evolution

The Kardashev scale is a method of measuring a civilization's level of technological advancement based on the amount of energy it is capable of using. The measure was proposed by Soviet astronomer Nikolai Kardashev (1932–2019) in 1964 and was named after him.

<span class="mw-page-title-main">X-ray binary</span> Class of binary stars

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor, to the other component, called the accretor, which is either a neutron star or black hole. The infalling matter releases gravitational potential energy, up to 30 percent of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.

Stellar radio sources, radio source stars or radio stars are stellar objects that produce copious emissions of various radio frequencies, whether constant or pulsed. Radio emissions from stars can be produced in many varied ways.

<span class="mw-page-title-main">Fermi Gamma-ray Space Telescope</span> Space telescope for gamma-ray astronomy launched in 2008

The Fermi Gamma-ray Space Telescope, formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor, is being used to study gamma-ray bursts and solar flares.

<span class="mw-page-title-main">Pulsar</span> Highly magnetized, rapidly rotating neutron star

A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission. Neutron stars are very dense and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays.

<span class="mw-page-title-main">Astrophysical jet</span> Beam of ionized matter flowing along the axis of a rotating astronomical object

An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as extended beams along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.

GCRT J1745−3009 is a Galactic Center radio transient (GCRT), or bursting low-frequency radio source which lies in the direction of the Galactic Center.

<span class="mw-page-title-main">Astropulse</span> BOINC based volunteer computing SETI@home subproject

Astropulse is a volunteer computing project to search for primordial black holes, pulsars, and extraterrestrial intelligence (ETI). Volunteer resources are harnessed through Berkeley Open Infrastructure for Network Computing (BOINC) platform. In 1999, the Space Sciences Laboratory launched SETI@home, which would rely on massively parallel computation on desktop computers scattered around the world. SETI@home utilizes recorded data from the Arecibo radio telescope and searches for narrow-bandwidth radio signals from space, signifying the presence of extraterrestrial technology. It was soon recognized that this same data might be scoured for other signals of value to the astronomy and physics community.

Rotating radio transients (RRATs) are sources of short, moderately bright, radio pulses, which were first discovered in 2006. RRATs are thought to be pulsars, i.e. rotating magnetised neutron stars which emit more sporadically and/or with higher pulse-to-pulse variability than the bulk of the known pulsars. The working definition of what a RRAT is, is a pulsar which is more easily discoverable in a search for bright single pulses, as opposed to in Fourier domain searches so that 'RRAT' is little more than a label and does not represent a distinct class of objects from pulsars. As of March 2015 over 100 have been reported.

<span class="mw-page-title-main">Gravitational wave</span> Propagating spacetime ripple

Gravitational waves are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves.

<span class="mw-page-title-main">Gamma-ray astronomy</span> Observational astronomy performed with gamma rays

Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy.

<span class="mw-page-title-main">Astrophysical X-ray source</span> Astronomical object emitting X-rays

Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.

RRAT J1819-1458 is a Milky Way neutron star and the best studied of the class of rotating radio transients (RRATs) first discovered in 2006.

<span class="mw-page-title-main">Fast radio burst</span> Astronomical high energy transient pulse

In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond, for an ultra-fast radio burst, to 3 seconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB releases as much energy in a millisecond as the Sun puts out in three days. While extremely energetic at their source, the strength of the signal reaching Earth has been described as 1,000 times less than from a mobile phone on the Moon.

<span class="mw-page-title-main">TON 618</span> Quasar and Lyman-alpha blob in the constellation Canes Venatici

TON 618 is a hyperluminous, broad-absorption-line, radio-loud quasar and Lyman-alpha blob located near the border of the constellations Canes Venatici and Coma Berenices, with the projected comoving distance of approximately 18.2 billion light-years from Earth. It possesses one of the most massive black holes ever found, at 40.7 billion M☉.

References

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