A fast radio burst (FRB) is a high energy burst of radio a few milliseconds long. Only a small number have been recorded, i.e., hundreds (from about 100 sources) through 2019, the first burst (the Lorimer burst, FRB 010724) having been recorded in 2001 and its existence uncovered in 2007. The origin of FRBs is under investigation, some progress recently made from a few bursts that irregularly repeat, but generally corresponding signals that might help identify and explain sources haven't been available. However, characteristics of the signals suggest 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. A number of theories have been put forward, such as neutron star quakes or superluminous supernovae. The antennas that have caught them have isolated them to arcminutes, but their direction needs to be identified to arcseconds to identify a galaxy or other source. Given observation counts and the fields of view that yielded the observations, it has been estimated that their rate across the sky is on the order of 5000 per day.
Among the early bursts were some that are now discounted. They had been labeled perytons and strongly suspected of being unreal, and later where shown to be from opening the door of a nearby microwave oven. An early claim was that one (non-peryton) burst came from M87, but this has been discounted. Some bursts have been observed by multiple telescopes, establishing their identity as astronomical. FRB121102 was the first to be followed up by additional bursts from the same direction (non-periodically), yielding a means of more detailed study and a galaxy has been identified as its source, and since 2018, more repeaters have been found. Additional efforts to capture them, including more radio telescopes (CHIME, FAST) have led to more information, such as the presence of changes within a single burst to lower frequencies, even when discounting the obvious travel-induced dispersion. They show a pattern nicknamed sad trombone is 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 could be fast optical bursts, (FOBs) associated with them. Unfortunately, they 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 the LSST.
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, and are 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 in its infancy, and as of 2019, major budgeting for FRB detection is just beginning. It 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.