In radio astronomy, a fast radio burst (FRB) is a high-energy astrophysical phenomenon of unknown origin manifested as a transient radio pulse lasting only a few milliseconds.
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Features
These are bright, unresolved, broadband, millisecond flashes found in parts of the sky outside the Milky Way.
Frequencies and dispersion
The component frequencies of each burst are delayed by different amounts of time depending on the wavelength. This delay is described by a value referred to as a dispersion measure.
Extragalactic origin
Fast radio bursts have pulse dispersion measures which are much larger than expected for a source inside the Milky Way; and consistent with propagation through an ionized plasma, and furthermore their distribution is isotropic (not especially coming from the galactic plane), thus they are conjectured to be of extragalactic origin.
Nomenclature
Fast radio bursts are named by the date the signal was recorded, as "FRB YYMMDD". The first fast radio burst to be described, the Lorimer Burst FRB 010724, was identified in 2007 in archived data recorded by the Parkes Observatory on 24 July 2001. Since then, most known FRBs have been found in previously recorded data. On 19 January 2015, astronomers at Australia's national science agency (CSIRO) reported that a fast radio burst had been observed for the first time live, by the Parkes Observatory.
Lorimer Burst
The Lorimer Burst (FRB 010724) was discovered in 2007 in archived data taken in 2001 by the Parkes radio dish in Australia. Analysis of the survey data found a 30-jansky dispersed burst which occurred on 24 July 2001, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud. The reported burst properties argue against a physical association with the Milky Way galaxy or the Small Magellanic Cloud. The burst became known as the Lorimer Burst. The discoverers 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 merger of relativistic objects. It is suggested that hundreds of similar events could occur every day and, if detected, could serve as cosmological probes.
Further developments
In 2010 there was a report of 16 similar pulses: clearly of terrestrial origin; detected by the Parkes radio telescope; and given the name perytons. In 2015 perytons were shown to be generated when microwave oven doors were suddenly opened during a heating cycle, with emission generated by the magnetron.
An observation in 2012 of a fast radio burst (FRB 121102) in the direction of Auriga in the northern hemisphere using the Arecibo radio telescope confirmed the extragalactic origin of fast radio pulses by an effect known as plasma dispersion. Victoria Kaspi of the McGill University estimates that as many as 10,000 fast radio bursts may occur per day over the entire sky.
In 2013 four bursts were identified that supported the likelihood of extragalactic sources.
FRB 140514, caught 'live' in 2014, was found to be 21% (+/- 7%) circularly polarised.
Fast radio bursts discovered up until 2015 had dispersion measures that were close to multiples of 187.5 cm−3 pc. However subsequent observations do not fit this pattern.
In 2015, FRB 110523 was discovered in archival data from the Green Bank Telescope. It was the first FRB for which linear polarization was detected (allowing, with the detection of circular polarisation, a calculation of Faraday rotation). Measurement of the signal's dispersion delay suggested that this burst is of extragalactic origin, possibly up to 6 billion light years away.
The upcoming and unusual Canadian radio telescope called CHIME will also be used to detect "hundreds" of fast radio bursts as its secondary objective.
FRB 150418
On 18 April 2015, FRB 150418 was detected by the Parkes observatory and within hours, several telescopes including the Australia Telescope Compact Array caught an "afterglow" of the flash, which took six days to fade. The Subaru telescope was used to find what was thought to be the host galaxy and determine its redshift and the implied distance to the burst.
However, the origin of the burst was soon disputed, and by April 2016 it was established that the emission instead originates from an active galactic nucleus that is powered by a supermassive black hole with dual jets blasting outward from the black hole. It was also noted that what was thought to be an "afterglow", never goes away, meaning that it cannot be associated with the fast radio burst.
FRB 121102
In November 2015, astronomer Paul Scholz at McGill University in Canada, found ten non-periodically repeated fast radio pulses in archival data gathered in May and June 2015 by the Arecibo radio telescope. The ten bursts have dispersion measures and sky positions consistent with the original burst FRB 121102, detected in 2012. Like the 2012 burst, the 10 bursts have three times the maximum plasma dispersion measure from a source in the Milky Way Galaxy. The team thinks that this finding rules out self-destructive, cataclysmic events that could only occur once, such as the explosion of a black hole or the collision between two neutron stars. According to the scientists, the data support an origin in a young rotating neutron star (pulsar), or in a highly magnetized neutron star (magnetar), or from highly magnetized pulsars travelling through asteroid belts, or from an intermittent Roche-lobe overflow in a neutron star-white dwarf binary.
On December 16, 2016 6 new FRBs were reported in the same direction. This is the only known instance in which these signals have been found twice in the same location in space. FRB 121102 is located at a minimum distance of ∼1150 AU away from earth, and is almost certainly extragalactic in nature. As of January 2017, FRB 121102 is believed to be co-located in a dwarf galaxy about three billion light-years from earth with a low-luminosity active galactic nucleus, or a previously unknown type of extragalactic source, or a young neutron star energizing a supernova remnant.
Origin Hypotheses
Because of the isolated nature of the observed phenomenon, the nature of the source remains speculative. As of 2016, there is no generally accepted explanation. The emission region is estimated to be no larger than a few hundred kilometers (because of causality). If the bursts come from cosmological distances, their sources must be very bright.
One possible explanation would be a collision between very dense objects like merging black holes or neutron stars. It has been suggested that there is a connection to gamma-ray bursts. Some have speculated that these signals might be signs of extraterrestrial intelligence.
In 2007, just after the publication of the e-print with the first discovery, it was proposed that fast radio bursts could be related to hyperflares of magnetars. In 2015 three studies supported the magnetar hypothesis.
Blitzars were proposed in 2013 as an explanation. In 2014 it was suggested that following dark matter-induced collapse of pulsars, the resulting expulsion of the pulsar magnetospheres could be the source of fast radio bursts. In 2016 the collapse of the magnetospheres of Kerr-Newman black holes are proposed to explain the origin of the FRBs' "afterglow" and the weak gamma-ray transient 0.4 s after GW 150914. It has also been proposed that if fast radio bursts originate in black hole explosions, FRBs would be the first detection of quantum gravity effects.