Fast Radio Bursts: Observations & Theories

• Fast Radio Bursts are millisecond-duration extragalactic transients characterized by extreme brightness temperatures, large dispersion measures, and diverse polarization properties.
• Advanced detection and precise localization techniques using interferometry and multi-wavelength observations have refined our understanding of FRB populations and their environments.
• FRBs serve as powerful cosmological probes by mapping intergalactic baryon content, testing compact object theories, and constraining fundamental physics through gravitational lensing and timing analyses.

Fast radio bursts (FRBs) are extragalactic millisecond-duration radio transients characterized by extremely high brightness temperatures, large dispersion measures, and apparent cosmological distances. Since their discovery in 2007, FRBs have become a central topic in time-domain astronomy and high-energy astrophysics, offering opportunities for studying compact object physics, probing the ionized cosmic web, and testing fundamental physical theories. Despite extensive observational and theoretical advances, the nature of their progenitors and the emission mechanism remain active areas of research.

1. Defining Phenomenology and Observational Properties

FRBs are defined by several robust observational characteristics:

• Duration and Flux: FRBs are single radio pulses with durations ranging from fractions of a millisecond (as short as 60 ns for some bursts) up to ~30 ms, with peak flux densities spanning 0.1 Jy to >100 Jy (Lorimer et al., 29 May 2024, Nimmo et al., 2021).
• Dispersion Measure (DM): The observed arrival time delay is frequency-dependent, following

$$\Delta t \simeq 4150\ \mathrm{s}\, \left[ (\nu_{\rm lo}/\mathrm{MHz})^{-2} - (\nu_{\rm hi}/\mathrm{MHz})^{-2} \right] \left( \frac{\mathrm{DM}}{\mathrm{cm}^{-3}\,\mathrm{pc}} \right)$$

where $\nu_{\mathrm{lo}}$ and $\nu_{\mathrm{hi}}$ are observed frequencies. Measured DMs typically range from several hundred up to more than 2600 pc cm⁻³ and are often in excess of the expected Galactic contribution, indicating an extragalactic origin (Lorimer et al., 29 May 2024, Marcote et al., 2019).

• Brightness Temperature: Derived values exceed $10^{35}$–$10^{41}$ K, implying emission must be coherent (Popov et al., 2018, Nimmo et al., 2021).
• Event Rate and Sky Distribution: The all-sky rate is several thousand per day, with isotropic sky distribution (Lorimer et al., 29 May 2024, Popov et al., 2018). As of September 2024, over 800 distinct FRB sources and more than 7600 bursts (from 67 repeaters) have been cataloged (Wu et al., 20 Sep 2024).
• Repeaters vs. Non-Repeaters: A minority of sources, such as FRB 121102, produce repeated bursts, some with hundreds to thousands of detected events (Hessels, 2018, Wu et al., 20 Sep 2024).

The cosmological distances are confirmed via DMs, host galaxy identifications (e.g., the first repeater, FRB 121102, localized to a dwarf galaxy at z=0.19), and the empirical Macquart relation between DM and redshift (Lorimer et al., 29 May 2024, Marcote et al., 2019). A central role is played by polarization and Faraday rotation; some FRBs exhibit nearly 100% linear (Marcote et al., 2019) or prominent circular polarization components (Feng et al., 2022), with rotation measures (RM) up to ≳10⁵ rad m⁻² in highly magnetized environments.

2. Physical Mechanisms and Progenitor Models

Theoretical efforts focus on compact object models—either cataclysmic or repeating. Two dominant classes are:

• Magnetar Models: Young, highly magnetized neutron stars (surface fields ∼10¹⁴–10¹⁵ G) are favored, particularly due to the observed properties of repeaters, strong polarization, and the association of at least one Galactic FRB (FRB 20200428A) with an X-ray outburst from SGR 1935+2154 (Xiao et al., 2022, Bailes, 2022). Emission mechanisms include:
  • Magnetospheric Coherent Curvature Radiation: Charged particle bunches moving along curved magnetic field lines produce coherent emission at GHz frequencies (Xiao et al., 2022). The differential power spectrum for a bunch is

  $$\frac{dP}{d\omega} \propto \frac{\sqrt{3}e^2 \gamma}{2\pi \rho} \left( \frac{\omega}{\omega_{\mathrm{c}}} \right) \int_{\omega/\omega_{\mathrm{c}}}^{\infty} K_{5/3}(y) dy$$

  where $\gamma$ is Lorentz factor, $\rho$ radius of curvature, $K_{5/3}$ a modified Bessel function, and the total power scales as $N_e^2$ for a bunch of $N_e$ electrons (coherence limit) (Xiao et al., 2022). - Synchrotron Maser Emission from Magnetized Shocks: Outward-propagating flares interact with surrounding plasma, producing shocks in which the synchrotron maser mechanism operates (Popov et al., 2018, Xiao et al., 2022). The characteristic frequency depends on Lorentz factor and field strength.

• Pulsar Giant Pulse (“Supergiant Pulse”) Models: Fast, energetic, "nanoshot" pulses observed from young Galactic pulsars (e.g., the Crab) suggest an energetic, possibly scaled-up version could explain FRBs (Popov et al., 2018, Muñoz et al., 2019, Nimmo et al., 2021). The link is bolstered by observations of FRB 20200120E, whose bursts bridge the luminosity and timescale gap between Galactic giant pulses and typical extragalactic FRBs (Nimmo et al., 2021).
• Cataclysmic Models: Proposed for non-repeating FRBs, e.g., mergers of compact object binaries, neutron star–black hole collisions (where Lense–Thirring–enhanced spin-up can drive giant pulses (Bhattacharyya, 2017)), and crust collapse in strange stars (Zhang et al., 2018).

More exotic proposals include neutron star–asteroid impacts (Geng et al., 2015), FRBs in AGN accretion disks (Zhao et al., 5 Mar 2024), or collapse of supramassive neutron stars (“blitzar” models) (Petroff et al., 2019). The field remains unsettled as evidence accumulates for both repeating and “one-off” FRB channels.

3. Statistical Properties and Population Demographics

Large samples have enabled detailed population analyses:

• Energy Distributions: Burst energies follow either a (broken) power law,

$\frac{dN}{dE} \propto E^{-\alpha_E}$

with possible log-normal or Cauchy components for repeaters (Wu et al., 20 Sep 2024). Break energies and indices differ for low- and high-fluence events; distributions sometimes show bimodality.

• Waiting Time Distributions: The interval $\Delta T = T_{i+1} - T_i$ between consecutive bursts for repeaters is inconsistent with a stationary Poisson process. The observed distribution is best fit by a superposition of exponentials,

$$P(\Delta t) = \sum_i \phi_i \lambda_i \exp(-\lambda_i \Delta t)$$

or by a Weibull function, revealing clustering and memory effects (see below) (Wu et al., 20 Sep 2024, Wang et al., 2023). Bimodal waiting-time structure in some repeaters reflects multi-timescale activity.

• Host Galaxy Demographics: Hosts range from low-metallicity dwarfs to more typical spiral galaxies, but repeaters show a tendency toward star-forming, sometimes extreme, environments (Hessels, 2018, Marcote et al., 2019).

Ongoing efforts focus on refining selection-bias corrections, energy function shape, the true repeater fraction, and local environment properties via subarcsecond-milliarcsecond localization (Marcote et al., 2019).

4. Polarization, Memory, and Intrinsic Source Activity

FRB polarization and temporal behavior provide essential constraints on emission mechanisms and environments:

• Polarization: A subset of FRBs exhibit high degrees of linear or circular polarization (Feng et al., 2022). The detection of up to 64% circular polarization in repeaters implies significant Faraday conversion or rare intrinsic emission conditions; Faraday conversion in highly magnetized, inhomogeneous plasma is favored in some FAST observations, but magnetospheric origin is also plausible (Feng et al., 2022). High and variable RMs (up to $10^5\ \mathrm{rad}\ \mathrm{m}^{-2}$) are found in repeaters with strong local magnetic fields, e.g., in the vicinity of massive black holes or nebulae (Marcote et al., 2019).
• Burst “Memory” and Correlation: Analysis of FRB burst sequences, especially in repeaters, demonstrates “memory” effects—bursts cluster in time, with coherent growth in burst rate and power-law waiting time tails. Quantification via the Hurst exponent ($H\sim0.62\mbox{--}0.70$) confirms persistent, long-term correlations inconsistent with simple Poisson processes (Wang et al., 2023). These can be reproduced using self-organized criticality models, favoring scenarios such as magnetar crustal failure and avalanche triggering via magnetic stress redistribution.

This evidence points to complex, correlated source dynamics and rules out purely random triggering in the central engine of repeating FRBs.

5. Cosmological Applications and Large-scale Structure Probing

FRBs, due to their extragalactic distances and strong propagation effects, serve as unique cosmological probes:

• Baryon Census (“Missing Baryons”): The average intergalactic DM can be written as (Wu et al., 20 Sep 2024)

$$\langle \mathrm{DM}_{\mathrm{IGM}}(z) \rangle = \frac{3c\,\Omega_b H_0}{8\pi G m_p} \int_0^z \frac{f_{\mathrm{IGM}}(z') f_e(z') (1+z')}{\sqrt{\Omega_m (1+z')^3+\Omega_\Lambda}}\, dz'$$

allowing the determination of the cosmic baryon fraction, mapping the "missing baryons" in the warm-hot IGM.

• Circumgalactic Medium (CGM) Probing: Comparison of DMs for FRBs with/without foreground galaxy crossings statistically reveals the electron content of the CGM (order of ∼90 pc cm⁻³ per crossing) (Wu et al., 20 Sep 2024).
• Cosmic Reionization and Cosmological Parameters: At $z>5$, the redshift–DM relation under changing ionization fraction $x_i(z)$ of hydrogen and helium can be used to paper the epoch of reionization (Wu et al., 20 Sep 2024). Localized FRBs (with host redshift) enable joint fits to measure the Hubble constant $H_0$, $\Omega_b$, and dark energy equation of state $w$.
• Gravitational Lensing: FRBs lensed by intervening masses yield multiple copies with measurable time delays ($\Delta t$), scaling as $$\Delta t \sim 1.97\times10^{-5}\ \mathrm{s}\ (1+z_l)(M/M_\odot)$$ for a point-mass lens (Pastor-Marazuela, 2 Dec 2024). This renders FRBs unique probes for constraining lens population, measuring $H_0$, and testing the compact object content of dark matter (Pastor-Marazuela, 2 Dec 2024, Wu et al., 20 Sep 2024).

Key challenges include disentangling the various DM contributions (Milky Way, host galaxy, local source), modeling inhomogeneities in the IGM, and securing precise redshifts through localization campaigns.

6. Instrumentation, Detection, and Future Directions

Technological innovation is central to FRB science:

• Detection: Modern facilities—CHIME/FRB, ASKAP, MeerKAT, FAST, DSA-2000, and others—employ wide fields of view, real-time detection pipelines, and, increasingly, raw voltage capture to preserve temporal and phase structure for post-facto analysis (Pastor-Marazuela, 2 Dec 2024).
• Localization: Milliarcsecond interferometric astrometry is essential for host galaxy association, environmental studies, and for robust lensing/cosmological applications (Marcote et al., 2019, Pastor-Marazuela, 2 Dec 2024).
• Multi-wavelength Campaigns: Coordinated gamma-ray (e.g., VERITAS) and X-ray and optical observations test for high-energy counterparts (Holder et al., 2019), with upper limits constraining emission models.
• Observational Prospects: Next-generation facilities (CHORD, SKA, DSA-2000) are expected to increase detection rates by at least an order of magnitude, with improvements in time and frequency resolution, polarization calibration, and DM precision (Pastor-Marazuela, 2 Dec 2024). Strategic advances in real-time raw data buffering and high time resolution search pipelines (down to nanoseconds) are being implemented (Nimmo et al., 2021, Pastor-Marazuela, 2 Dec 2024).

Anticipated future developments include expanding sample sizes for population statistics, identifying gravitationally lensed FRBs for cosmology, uncovering the physics of ultra-fast transients, and clarifying the connection between repeating and non-repeating progenitors.

7. Outstanding Challenges and Open Questions

Despite significant progress, multiple open questions persist:

• What fraction of FRBs repeat, and do repeaters and non-repeaters represent distinct astrophysical classes or a continuum?
• What is the physical mechanism enabling coherent GHz emission with such high brightness temperatures, and under what conditions are circular polarization or extreme RMs produced (Feng et al., 2022)?
• How do various progenitor models (e.g., magnetars, young pulsars, mergers, accretion-driven events in AGN disks) populate the observed phenomenological space (Zhao et al., 5 Mar 2024)?
• How can selection biases, host galaxy DMs, and environmental propagation effects be robustly modeled to enable FRBs as precision cosmological tools (Wu et al., 20 Sep 2024)?
• What is the role of environmental feedback (e.g., cavities in AGN disks), multi-timescale “memory” effects, and self-organized criticality in driving repeating burst activity (Wang et al., 2023)?

Future systematic monitoring, wide-field and high-cadence surveys, advanced polarization and spectral analysis, and multi-messenger (gravitational wave, neutrino) searches are expected to resolve these questions and solidify the role of FRBs in both astrophysical and cosmological research.