Astronomical radio source

Astronomical radio sources are objects in outer space that emit strong radio waves. Radio emission comes from a wide variety of sources. Such objects represent some of 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.^[1] 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.^[2]

Sources

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]

The galactic center

The galactic center of the Milky Way was the first radio source to be detected. It contains a number of radio sources, including Sagittarius A* and the supermassive black hole at its center.

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.

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.

Supernovas 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.

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.^[4]

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 red shift led us to conclude that they are distant active galactic nuclei. 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.^[5]

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,^[6] that was the beginning of the universe).

Primordial black holes

According to the Big Bang Model (also called the Standard 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^[7] 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.^[8]

RRATs

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.^[9] Early efforts have been able to detect radio emissions (sometimes called RRAT flashes)^[10] 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.

Extragalactic pulses

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.^[11] 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.^[12] 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,^[13] but in 2013 four pulse sources were identified that supported the likelihood of a genuine extragalactic pulsing population.^[14]

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.

ET

Previous searches by SETI@home 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 as complimentary to the narrow-band SETI@home survey as a by-product of the search for physical phenomena.

Undiscovered phenomena

Explaining their recent discovery of a powerful bursting radio source, NRL astronomer Dr. Joseph Lazio stated:^[15] "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

• Active galactic nucleus
• Astrometry
• Electromagnetic radiation
• Radio astronomy
• Astrophysical X-ray source

References

1. Koupelis, Theo; Karl F. Kuhn (2007). In Quest of the Universe (5th ed.). Jones & Bartlett Publishers. p. 149. ISBN 0-7637-4387-9. Retrieved 2008-04-02.
2. Braes, L.L.E. (1974). "Radio Continuum Observations of Stellar Sources". IAU Symposium No.60, Maroochydore, Australia, September 3–7, 1973. 60: 377–381. Bibcode:1974IAUS...60..377B.
3. Michael Stix (2004). The sun: an introduction. Springer. ISBN 978-3-540-20741-2. section 1.5.4 The Radio Spectrum
4. Gonzalez, Guillermo; Wesley Richards (2004). The Privileged Planet. Regnery Publishing. p. 382. ISBN 0-89526-065-4. Retrieved 2008-04-02.
5. "Archived copy". Archived from the original on 2006-01-28. Retrieved 2006-03-29.
6. "Cosmic Detectives". The European Space Agency (ESA). 2013-04-02. Retrieved 2013-04-26.
7. "The case for mini black holes". Cern Courier. 2004-11-24. Retrieved 2010-06-23.
8. "Primordial Black Holes". SETI@home. Retrieved 2010-06-23.
9. David Biello (2006-02-16). "New Kind of Star Found". Scientific American. Retrieved 2010-06-23.
10. Jodrell Bank Observatory. "RRAT flash". Physics World. Retrieved 2010-06-23.
11. D. R. Lorimer; M. Bailes; M. A. McLaughlin; D. J. Narkevic; F. Crawford (2007-09-27). "A Bright Millisecond Radio Burst of Extragalactic Origin". Science. Retrieved 2010-06-23.
12. Duncan Lorimer (West Virginia University, USA); Matthew Bailes (Swinburne University); Maura McLaughlin (West Virginia University, USA); David Narkevic (West Virginia University, USA) & Fronefield Crawford (Franklin & Marshall College, USA) (October 2007). "A bright millisecond radio burst of extragalactic origin". Australia Telescope National Facility. Retrieved 2010-06-23.
13. Sarah Burke-Spolaor; Matthew Bailes; Ronald Ekers; Jean-Pierre Macquart; Fronefield Crawford III (2010). "Radio Bursts with Extragalactic Spectral Characteristics Show Terrestrial Origins". arXiv:1009.5392v1 [astro-ph.CO].
14. D. Thornton; B. Stappers; M. Bailes; B. Barsdell; S. Bates; N. D. R. Bhat; M. Burgay; S. Burke-Spolaor; D. J. Champion; P. Coster; N. D'Amico; A. Jameson; S. Johnston; M. Keith; M. Kramer; L. Levin; S. Milia; C. Ng; A. Possenti; W. van Straten (2013-07-05). "A Population of Fast Radio Bursts at Cosmological Distances". Science. Retrieved 2013-07-05.
15. Andrea Gianopoulos; Shannon Wells; Michelle Lurch-Shaw; Janice Schultz; DonnaMcKinney (2005-03-02). "Astronomers Detect Powerful Bursting Radio Source Discovery Points to New Class of Astronomical Objects". Retrieved 2010-06-23.