A fast radio burst (FRB) is a high energy burst of radio a few milliseconds long. As of 2021, thousands have been recorded, from thousands of sources, largely by radio telescopes adapted to detect them. The first burst (the Lorimer burst, FRB 010724) was recorded in 2001 and this record noticed in 2007, and for some years afterward, no more than a few were detected. Some bursts were observed by multiple telescopes, establishing their identity as astronomical, and characteristics of the signals indicate they are from beyond the Milky Way: the dispersion measures (DM) indicate they passed through more electrons than can be accounted for by the Milky-Way interstellar medium. The origin of FRBs remains a topic of research, with numerous theories, including as neutron star quakes or superluminous supernovae, and there remains the possibility of more than one mechanism. Given current FRB observation counts and the fields of view that yield these counts, their rate across the sky is thought to be thousands per day.
Among the early recorded bursts were some termed perytons that were strongly suspected of being unreal, and indeed were shown to result from opening the door of a nearby microwave oven. FRB121102 was the first FRB to be followed up by additional bursts from the same direction (non-periodically), allowing detailed study including identifying a source galaxy, and since 2018, more such repeaters have come to light. Additional efforts to record FRBs, including more radio telescopes (CHIME, FAST) have led to more information, such as the presence of shifts within a single burst to lower frequencies, even when discounting the obvious travel-induced dispersion. They show a pattern nicknamed sad trombone, incorporating several sub-bursts, each with a downward-sliding frequency and each starting at a lower frequency that the previous. Theories based upon repeating FRBs suggest there might exist fast optical bursts, (FOBs) associated with them. Unfortunately, FOBs are presumed to be sufficiently dim that catching one could require continuous monitoring of an established source-location by a major research telescope and such an FOB would be much too short to be caught by VRO's normal cadence.
FRBs have potential as probes of the intergalactic medium (IGM) and their host galaxy. An observed DM can be compared with models that calculate the expected contribution of the Milky Way, the IGM, and the FRB's host galaxy: if there are good independent estimates for two of those, the remainder is a possibly-useful estimate of the third. Another characteristic of interest is that some of the pulses are somewhat longer and tail off, presumed to have undergone a process termed scatter broadening, and a plausible explanation of the differing types is that it stems from characteristics of the host galaxy. The bursts are subject to gravitational lensing, and at least one recorded burst apparently traversed the halo of an intervening galaxy.
The term fast radio transient (FRT) means basically the same thing as FRB, but also includes any abrupt beginning or end of a lengthier signal. FRB astronomy is still relatively new but has potential as part of the future's multi-messenger astronomy. It also has the potential to be used as a probe, such as analysis of an FRB that passes through halo of an intervening galaxy, and also the possibility of gravitational microlensing.
FRBs are cataloged online in the Transient Name Server. FRBCAT (aka FRB Catalogue) is an older online catalog of FRBs and candidates, no longer updated.