Fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond. FRB are also bright, unresolved (pointsource-like), broadband (spanning a large range of radio frequencies), millisecond flashes found in parts of the sky. Unlike many radio sources, the signal from a burst is detected in a short period of time with enough strength to stand out from the noise floor. Fast radio bursts have pulse dispersion measurements > 100 pc cm−3.
The burst usually appears as a single spike of energy without any change in its strength over time. The bursts come from all over the sky, and are not concentrated on the plane of the Milky Way. Known FRB locations are biased by the parts of the sky that the observatories can image.
Many have radio frequencies detected around 1400 MHz; a few have been detected at lower frequencies in the range of 400–800 MHz. 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 (DM). This results in a received signal that sweeps rapidly down in frequency, as longer wavelengths are delayed more.
Astronomers estimate the average FRB releases as much energy in a millisecond as the sun puts out in 3 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. The first FRB was discovered by Duncan Lorimer and his student David Narkevic in 2007 when they were looking through archival pulsar survey data, and it is therefore commonly referred to as the Lorimer Burst.
Many FRBs have since been recorded, including several that have been detected to repeat in seemingly irregular ways. Nonetheless, one FRB has been detected to repeat in a regular way: particularly, FRB 180916 seems to pulse every 16.35 days. Most FRBs are extragalactic, but the first Milky Way FRB was detected by the CHIME radio telescope in April 2020.
When the FRBs are polarized, it indicates that they are emitted from a source contained within an extremely powerful magnetic field. The exact origin and cause of the FRBs is still the subject of investigation; proposals for their origin range from a rapidly rotating neutron star and a black hole, to extraterrestrial intelligence.
In 2020, astronomers reported narrowing down the source of fast radio bursts, which may now plausibly include “compact-object mergers and magnetars arising from normal core collapse supernovae”.
Because of the isolated nature of the observed phenomenon, the nature of the source remains speculative. As of 2020, there is no generally accepted single explanation, although a magnetar has been identified as a possible source. The sources are thought to be a few hundred kilometers or less in size, as the bursts last for only a few milliseconds, and if the bursts come from cosmological distances, their sources must be very energetic, generating as much energy in a millisecond burst as the Sun does in 80 years.
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 artificial in origin, that they may be signs of extraterrestrial intelligence.