Anastronomical radio source is an object inouter space that emits strongradio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in theuniverse.
In 1932, Americanphysicist andradioengineerKarl Jansky detectedradio waves coming from an unknown source in the center of the Milky Waygalaxy. Jansky was studying the origins of radio frequency interference forBell 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 byGrote 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 uniquebinaryMWC 349.[2]
As the nearest star, theSun is the brightest radiation source in most frequencies, down to the radio spectrum at 300 MHz (1 m wavelength). When the Sun is quiet, thegalactic background noise dominates at longer wavelengths. Duringgeomagnetic storms, the Sun will dominate even at these low frequencies.[3]
Oscillation of electrons trapped in themagnetosphere 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 moonIo injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generatesAlfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through acyclotronmaser 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]
In 2021 news outlets reported that scientists, with theJuno spacecraft that orbits Jupiter since 2016, detected anFM radio signal from the moonGanymede at a location where the planet's magnetic field lines connect with those of its moon. According to the reports these were caused bycyclotron maser instability and were similar to bothWiFi-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]
Thecenter of the Milky Way was the first radio source to be detected. It contains a number of radio sources, includingSagittarius A, the compact region around thesupermassive black hole,Sagittarius A*, as well as the black hole itself. When flaring, theaccretion 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, andGCRT J1742–3001.[8] In addition, ASKAP J173608.2-321635, which was detected six times in 2020, may be a fourth GCRT.[9][8]
In 2021, astronomers reported the detection of peculiar, highlycircularlypolarized intermittent radio waves from near the Galactic Center whoseunidentified 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 often show diffuse radio emission. Examples includeCassiopeia A, the brightest extrasolar radio source in the sky, and theCrab Nebula.
Supernovae sometimes leave behind dense spinningneutron stars calledpulsars. They emit jets of charged particles which emitsynchrotron radiation in the radio spectrum. Examples include theCrab Pulsar, the first pulsar to be discovered. Pulsars andquasars (dense central cores of extremely distant galaxies) were both discovered by radio astronomers. In 2003 astronomers using theParkesradio telescope discovered two pulsars orbiting each other, the first such system known.
Rotating radio transients (RRATs) are a type of neutron stars discovered in 2006 by a team led byMaura McLaughlin from theJodrell Bank Observatory at theUniversity 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 calledRRAT 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.
Shortradio waves are emitted from complexmolecules in dense clouds ofgas wherestars are giving birth.
Spiral galaxies contain clouds ofneutral hydrogen andcarbon 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]
Many galaxies are strong radio emitters, calledradio galaxies. Some of the more notable areCentaurus A andMessier 87.
Quasars (short for "quasi-stellar radio source") were one of the first point-like radio sources to be discovered. Quasars' extremeredshift led us to conclude that they are distant active galactic nuclei, believed to be powered byblack holes.Active galactic nuclei have jets of charged particles which emitsynchrotron radiation. One example is3C 273, the optically brightest quasar in the sky.
Merginggalaxy clusters often show diffuse radio emission.[15]
The cosmic microwave background isblackbodybackground radiation left over from theBig Bang (the rapid expansion, roughly 13.8 billion years ago,[16] that was the beginning of theuniverse.
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 theSmall 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 asfast radio bursts (FRBs). The first observed burst has become known as theLorimer burst.Blitzars are one proposed explanation for them.
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 ofAstropulse 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-energygamma rays and particles. This fireball would interact with the surrounding magnetic field, pushing it out and generating radio waves.[22]
Previous searches by various "search for extraterrestrial intelligence" (SETI) projects, starting withProject Ozma, have looked for extraterrestrial communications in the form of narrow-band signals, analogous to our own radio stations. TheAstropulse 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]
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.
