Fast Radio Bursts

TL;DR

Fast Radio Bursts (FRB) are enigmatic, extremely bright millisecond radio transients whose coherent emission implies compact, energetic sources. The paper surveys discovery history, basic observables (DM, RM, polarization), energetics, and identifications, and reviews leading emission models—primarily synchrotron maser scenarios in magnetized environments and curvature radiation from coherently bunched charges—along with environmental inferences from RM and DM. A central tension is the coexistence of repeating and apparently non-repeating FRB, plus constraints from baseband observations that challenge some maser scenarios by restricting emission-region sizes. The work highlights rapidly evolving observational constraints (dynamic spectra, duty cycles, periodic activity windows) and emphasizes the need for next-generation facilities (e.g., SKA) to identify progenitors and plasma physics mechanisms driving FRB.

Abstract

Eighteen years after their discovery, the astronomical sources and radiation mechanisms of fast radio bursts remain mysterious. Their radiation is as bright as that of pulsars, with brightness temperatures as high as $\sim 10^{36}$ K, implying coherent emission, but the plasma physics that forms the coherent charge bunches, with net charges of order a Coulomb, is not understood. Some FRB have been identified with galaxies at redshifts of a few tenths, but one originated within a globular cluster in the galaxy M81 at a distance of 3.6 Mpc. A minority of FRB have been observed to repeat, in some cases thousands of times. The vast majority of FRB have not been observed to repeat, but it is not known if they are truly ``one-offs'' or repeat at unobservably long intervals. Some FRB originate within dense, rapidly varying, plasma environments, while others appear to be surrounded by high vacuum. Hypotheses for their sources include magnetars and black hole accretion discs.

Figures (6)

• Figure 1: Discovery of the first Fast Radio Burst: the received intensity of the "Lorimer burst" as a function of radio frequency and time. This observation was made at the Parkes Observatory in Australia, a 64 m diameter radio telescope. The data are only one bit deep---if the spectral flux density was above a threshold the narrow rectangle corresponding to that time and frequency is black, if below threshold it is white. The threshold was chosen so that detector and sky noise is above threshold in about half the time-frequency elements and below threshold in the other half. The dark curved path shows the burst. Its curvature indicates propagation delay in plasma between the emitter and the observer; higher frequencies have higher group velocity and arrive earlier. The adjoining white curves were added to emphasize the signal. The horizontal black line shows a narrow spectral band excluded because of electronics failure. The inset shows the frequency-integrated flux as a function of time, where the frequency-dependent (dispersion) delays have been removed; the intrinsic full width at half maximum of the burst was about 7 ms. Reproduced with permission from L07.
• Figure 2: De-dispersed dynamic spectra of bursts of four types, reproduced with permission from P21b. The first three sub-figures are apparent non-repeaters. The fourth is a repeater, and shows a drift to lower frequency, distinct from dispersion. This "sad trombone" effect is found for most repeating bursts, but is unusual for apparent non-repeaters.
• Figure 3: Fitted spectral parameter "running" vs. burst length for several hundred bursts (numbers in figure), with apparent non-repeaters ("one-offs") green, repeaters purple and orange. The distributions of each variable, separately, overlap, but in the two-dimensional space there is a clear separation. Reproduced with permission from P21b.
• Figure 4: Burst bandwidth vs. duration, with apparent non-repeaters ("one-offs") green, repeaters purple and orange. The distributions in the two-dimensional space are nearly disjoint. Reproduced with permission from P21b.
• Figure 5: $|\Delta\text{DM}|$vs.$\Delta t$ for intervals between successive bursts of FRB 20190520B on MJD 59373; data from Table S3 of AT23.